While almost all rulings from the ingame battle system have been removed, altered, or added on to, Species Clause is the only one that remains unchanged. Its main use is to keep players from cheesing matches with multiples of the same Pokemon, but as time has gone by, particularly with the introduction of more and more alternate forms and regional variants of existing Pokemon, I believe that the strict limitation on teambuilding enforced by Species Clause has overstayed its welcome. Despite the original purpose of Species Clause being to prevent identical Pokemon from cheesing matches, it is now instead preventing Pokemon that are nowhere near as comparable to one another from being used on the same team, simply because they share the same dex number.
A considerable majority of Pokemon with alternate forms that are neither cosmetic (or close enough) or in-battle transformations all have a wide disparity in all their defining factors as a Pokemon between niche, viability, and even general tiering. Think along the lines of the Deoxys forms, Kyurem, Necrozma (not Ultra), Shaymin, Zacian/Zamazenta, and almost every regional variant. Simply put, I don't think it makes sense to limit use of these Pokemon paired with other forms while simultaneously allowing things like Chansey + Blissey, Latios + Latias, Slowbro + Slowking, Venusaur + Vileplume, etc.
With the above in mind, there are effectively two paths you can go down, between either loosening restrictions on teambuilding or restricting it further. The latter of these options isn't really ideal, as it involves the drawing of many subjective lines between how "similar" a mon has to be to another existing mon before being restricted, which only leaves the former being a potential viable option. When considering loosening the restrictions, you still have to be careful in order to avoid similar subjectivity, lest you otherwise risk opening the flood gates to things like double Keldeo, Magearna, Toxtricity, or even 6 Silvally.
The main solution I'm getting at is that Species Clause should be changed out for a new clause of similar effect that also allows a single regional variant to be used along with its base form, or vice versa.
The general problem with allowing all alternate forms of Pokemon to be used concurrently with one another is that you'd have to draw a line in the sand as to how "different" certain forms have to be from one another, lest they violate the original purpose of Species Clause, which becomes an especially awkward scenario for cases like Gourgeist and Pumpkaboo with notably different stats for each size, Basculin with different abilities, Rotom forms only changing type and one move, and Arceus/Silvally in general. Regional variants, on the other hand, is a much more easily defined group of Pokemon, thus cutting back on the issue of subjectivity.
With a considerable majority of all regional variants being in completely different worlds from one another, as far as niche, viability, and tiering goes, this stands to be a very minimal change to the dynamic of most metagames, which is exactly why I think it should be implemented, as it only serves to increase the range of teambuilding capability for those looking to have fun with the game.
For clarity's sake, here's a list of each Pokemon with a regional form and their corresponding tiering and viability ranking in said tier:
While a notable number of these cases are particularly close to their corresponding base forms, in terms of tiering, it's still important that we assess what each Pokemon is worth in their corresponding meta, and what they can do. For instance, Rapidash and Ponyta have nearly nothing in common beyond identical stats and similar coverage moves with one another, Stunfisk performs a similar role to one another, but does so with near everything else being different (with base Stunfisk seeing almost zero usage), Persian is almost completely different from one another, etc. With that said, however, there are also some cases that are close in almost all facets, like Golem, Raticate, Raichu, etc, that could stand to be problematic. Additionally, I have not explored comparability across other metagames and doubles.
Overall, while there might be some negatives that result in particular metas suffering from particular mons, I believe the broader impact of this change would be a net positive for teambuilding. The particular mons can always be banned if problems arise. Ultimately, with things like Blissey + Chansey, Magnezone + Magneton, Latios + Latias, etc all coming back through DLC, I just don't see how it makes sense to simultaneously restrict things like Dugtrio + Dugtrio-A, Corsola + Corsola-G, Darmanitan + Darmanitan-G, etc.
A name for a clause like this could be something along the lines of "Form Clause", or even just altering the definition of the existing Species Clause, with a shorthand description being that it limits teams to solely one use of a species of Pokemon on a team at a time, with regional variants being counted as a proper separate species, rather than a duplicate mon or form change.
A considerable majority of Pokemon with alternate forms that are neither cosmetic (or close enough) or in-battle transformations all have a wide disparity in all their defining factors as a Pokemon between niche, viability, and even general tiering. Think along the lines of the Deoxys forms, Kyurem, Necrozma (not Ultra), Shaymin, Zacian/Zamazenta, and almost every regional variant. Simply put, I don't think it makes sense to limit use of these Pokemon paired with other forms while simultaneously allowing things like Chansey + Blissey, Latios + Latias, Slowbro + Slowking, Venusaur + Vileplume, etc.
With the above in mind, there are effectively two paths you can go down, between either loosening restrictions on teambuilding or restricting it further. The latter of these options isn't really ideal, as it involves the drawing of many subjective lines between how "similar" a mon has to be to another existing mon before being restricted, which only leaves the former being a potential viable option. When considering loosening the restrictions, you still have to be careful in order to avoid similar subjectivity, lest you otherwise risk opening the flood gates to things like double Keldeo, Magearna, Toxtricity, or even 6 Silvally.
The main solution I'm getting at is that Species Clause should be changed out for a new clause of similar effect that also allows a single regional variant to be used along with its base form, or vice versa.
The general problem with allowing all alternate forms of Pokemon to be used concurrently with one another is that you'd have to draw a line in the sand as to how "different" certain forms have to be from one another, lest they violate the original purpose of Species Clause, which becomes an especially awkward scenario for cases like Gourgeist and Pumpkaboo with notably different stats for each size, Basculin with different abilities, Rotom forms only changing type and one move, and Arceus/Silvally in general. Regional variants, on the other hand, is a much more easily defined group of Pokemon, thus cutting back on the issue of subjectivity.
With a considerable majority of all regional variants being in completely different worlds from one another, as far as niche, viability, and tiering goes, this stands to be a very minimal change to the dynamic of most metagames, which is exactly why I think it should be implemented, as it only serves to increase the range of teambuilding capability for those looking to have fun with the game.
For clarity's sake, here's a list of each Pokemon with a regional form and their corresponding tiering and viability ranking in said tier:
Green = 3+ tier separation or Unranked
Yellow = 2 tier separation
Orange = 1 tier separation
Red = 0 tier separation
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Gen 7 LC C)
-(Gen 7 LC Uber / Gen 8 LC Uber)------------------------ (Gen 7 LC B / Gen 8 LC Uber)
-(Gen 7 LC A+ / Gen 8 LC A+)---------------------------- (Unranked)
-(Gen 7 Ubers A / ZU B- / Gen 8 Ubers A / Unranked)----- (Gen 7 PU C+ / Gen 8 RU C / NU ~B)
-(Gen 7 LC B / Gen 8 LC C) (Unranked) ------------- (Unranked)
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Gen 7 LC B)
-(Gen 7 ZU A-)------------------------------------------ (Gen 7 UU B-)
-(Gen 7 ZU B+)------------------------------------------ (Gen 7 RU B-)
-(Unranked)---------------------------------------------
(Unranked)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 LC B+)
-(Gen 8 LC B)------------------------------------------- (Unranked)
-(Gen 8 LC C)------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Unranked)
-(Gen 7 ZU A- / Gen 8 PU A-)---------------------------- (Gen 7 PU A- / Gen 8 RU A+)
-(Gen 7 RU A / Gen 8 RU A)------------------------------ (Gen 7 OU C- / Gen 8 OU C)
-(Gen 7 ZU A+)------------------------------------------ (Gen 7 NU B)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 UU C)
-(Gen 8 RU B-)------------------------------------------ (Gen 8 PU C+)
-(Gen 8 UU A)------------------------------------------- (Gen 8 Ubers B+)
-(Gen 7 ZU B-)------------------------------------------ (Gen 7 PU C+)
-(Gen 7 ZU B)------------------------------------------- (Gen 7 PU A)
-(Gen 7 Unranked / Gen 8 PU B+)------------------------- (Gen 7 PU B+ / Gen 8 RU C / NU ~B)
-(Gen 8 RU B)------------------------------------------- (Gen 8 UU A-)
-(Gen 8 PU A-)------------------------------------------ (Gen 8 NU C+)
-(Gen 7 ZU A+)------------------------------------------ (Gen 7 ZU B-)
-(Gen 8 LC A)------------------------------------------- (Gen 8 LC B+)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 RU C / NU B)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 UU C- / NU A)
Yellow = 2 tier separation
Orange = 1 tier separation
Red = 0 tier separation
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Gen 7 LC C)
-(Gen 7 LC Uber / Gen 8 LC Uber)------------------------ (Gen 7 LC B / Gen 8 LC Uber)
-(Gen 7 LC A+ / Gen 8 LC A+)---------------------------- (Unranked)
-(Gen 7 Ubers A / ZU B- / Gen 8 Ubers A / Unranked)----- (Gen 7 PU C+ / Gen 8 RU C / NU ~B)
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Gen 7 LC B)
-(Gen 7 ZU A-)------------------------------------------ (Gen 7 UU B-)
-(Gen 7 ZU B+)------------------------------------------ (Gen 7 RU B-)
-(Unranked)---------------------------------------------
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 LC B+)
-(Gen 8 LC B)------------------------------------------- (Unranked)
-(Gen 8 LC C)------------------------------------------- (Unranked)
-(Unranked)--------------------------------------------- (Unranked)
-(Gen 7 ZU A- / Gen 8 PU A-)---------------------------- (Gen 7 PU A- / Gen 8 RU A+)
-(Gen 7 RU A / Gen 8 RU A)------------------------------ (Gen 7 OU C- / Gen 8 OU C)
-(Gen 7 ZU A+)------------------------------------------ (Gen 7 NU B)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 UU C)
-(Gen 8 RU B-)------------------------------------------ (Gen 8 PU C+)
-(Gen 8 UU A)------------------------------------------- (Gen 8 Ubers B+)
-(Gen 7 ZU B-)------------------------------------------ (Gen 7 PU C+)
-(Gen 7 ZU B)------------------------------------------- (Gen 7 PU A)
-(Gen 7 Unranked / Gen 8 PU B+)------------------------- (Gen 7 PU B+ / Gen 8 RU C / NU ~B)
-(Gen 8 RU B)------------------------------------------- (Gen 8 UU A-)
-(Gen 8 PU A-)------------------------------------------ (Gen 8 NU C+)
-(Gen 7 ZU A+)------------------------------------------ (Gen 7 ZU B-)
-(Gen 8 LC A)------------------------------------------- (Gen 8 LC B+)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 RU C / NU B)
-(Gen 8 NU Unranked)------------------------------------ (Gen 8 UU C- / NU A)
Overall, while there might be some negatives that result in particular metas suffering from particular mons, I believe the broader impact of this change would be a net positive for teambuilding. The particular mons can always be banned if problems arise. Ultimately, with things like Blissey + Chansey, Magnezone + Magneton, Latios + Latias, etc all coming back through DLC, I just don't see how it makes sense to simultaneously restrict things like Dugtrio + Dugtrio-A, Corsola + Corsola-G, Darmanitan + Darmanitan-G, etc.
A name for a clause like this could be something along the lines of "Form Clause", or even just altering the definition of the existing Species Clause, with a shorthand description being that it limits teams to solely one use of a species of Pokemon on a team at a time, with regional variants being counted as a proper separate species, rather than a duplicate mon or form change.