I frankly didn't like the Battle Frontier much either. Outside of the Battle Tower and Battle Factory (which I both really like; it's a shame the latter was also cut outside of USUM), the other facilities are too gimmicky to be enjoyable.
(Not to mention the Battle Factory was also a nice way to get BP when you didn't have a competitive team yet)
I actually sort of agree but with an asterisk. Probably be easier to do each Facility with my thoughts:
Tower: Nothing wrong here, is just plain ol' single and double battles.
Factory: Nothing wrong here either, infact it's probably the newbie friendliest as it's rental battles. Probably why it has appeared sporadically in some form while others were one and done (aside from Tower of course which has an expy every generation).
Arena: Not too bad but the luck factor here is strong if you don't get lucky with an opponent. Also its very offense focused (punishing you for even using Protect variants) and even has you playing safe (you don't want moves with imperfect accuracy as they deduct points for every move missed). Also the move coverage a Pokemon has the better if not recommended (super effective gets you points, resisted loses).
Dome: Just a straight-up knock-out tournament. I guess there's the additional mechanic of learning what your opponent's Pokemon and strategy which I guess suggests the AI will also try hard countering your choices... as if it didn't already in the Battle Tower, they're just admitting it here. So, yeah, I guess in that case it's no different from Single Battles in the Tower.
Pike: This one is interesting as I think it's entirely possible to go through a streak without ever battling (obviously on a streak where you face the Frontier Brain that's not true... just means you won't do any battles before facing Lucy). However it can just as easily screw you over by you choosing a hall that gives you a status ailment. And if you really get unlucky you won't get a healing trainer as your Pokemon don't heal after every battle. As the Symbol it represents, its a really luck based facility so I don't think it's unfair to not liking it.
Palace: Bleh, what your Pokemon decides to do is entirely based on what their Nature is and on RNG on top of that. Now all facilities requires breeding to a certain extent for good Pokemon to use in them, but this one especially limits you as you gotta find what Nature fits your battling style and then what Pokemon goes good with that Nature. This one can stay in Emerald, give Spenser a new gimmick if here ever comes back or at the very least simplify the mechanics so it makes (why does a Lonely, Jolly & Rash Pokemon want to support more? Why are Adamant & Modest average all-around? Why aren't the natures Defense & Special Defense focused have a heavier lean on using defensive moves?).
Pyramid: A really interesting idea, though maybe not as a battle facility. This is actually a maze to the top of the pyramid where you battle both trainers and wild Pokemon and can even find items like potions, ethers, restores, and held items (a Pokemon that knows Pickup is especially helpful as they can find more items for you). But this exploration and survival aspect is what makes it not that great of a battle facility as battle facilities should be more battle after battle where the gimmick changes how battles are proceed. Pyramid will make for a great side activity like the Vlack Tower or White Treehollow in B2W2.
The Gen IV Battle Frontier I think did a lot better with their Facilities.
I must admit that while I could see the merit in them, even when I was younger and had more time on my hands I didn't really bother with Platinum/HGSS's frontier much. It was always far too intimidating and felt like a level I could never reach and furthermore even in the Battle Tower I often lost for luck reasons outside of my control; and I can only imagine this is a big part of what contributed to GF realising not a lot of people played these. They're very interesting in concept, but in execution they need a bit more of a draw and something to ease people in. Maybe there are lower versions with less penalties, but then more 'competitive' levels the further in you go that much more resemble the Frontiers we're used to. A little bit like Battle Tree's current 'singles' and 'super singles', but a little bit more thought out.
Yeah, while I love the idea of the Battle Frontier it also does throw you into the deep end of the pool. It's why I think instead of having rentals only at one Facility they should all have rentals (just make it so you can't earn a Symbol/Print/whatever. Also the Battle Factory or its expy can still be included as it has the rental swapping mechanic it could even add onto). Or maybe even do what they revealed they're planning for SwSh and let us rent a team another player had used to beat a facility. It'll allow us to participate and see what goes into it first before we make our own team and strategy. Infact, maybe even help out there by having a Daycare and person that'll Hyper Train our Pokemon (even sell Silver Bottle Caps in the BP Shop).
As someone who loved the Battle Factory but wasn't that big a fan of the other formats, an easy fix would be to fold the rental system into the standard Battle Building.
And that's another thought, a lot of these gimmicks can be rolled into the Battle Tower expy with little difficulty. They don't need their entire own location, just a different mode. Don't get me wrong, I like each Facility was given its own flavor, but if they're that tight on time it might just be bet to keep what's tried and true and add on new ideas to that to see if anything sticks (for example I do think rentals, Inverse Battles, and maybe even Little Cup could be added to the Battle Tower expies as standard features for now on; and if this results in the initial trainer pool to be smaller they could always add in more via version updates).
Like, here's an idea: next Battle Frontier could be a TV Network. You go in, sign-up for what program you want to be on, and from there you battle according to the gimmick that program uses with the host of the program being the Frontier Brain. This would just mean the only designing they would need to do would be the TV Network main building (including where is the BP Shop, Daycare, Hyper Trainer, etc.; all which could also be their own programs) and then all the rooms where the programs are televised from (and the programs could be a news show, food show, weather report, shopping show, nature show, etc.). Maybe also have some additional programs or rooms connected to the main hall where you can talk to NPCs and get tips and helpful advice.
New pre-evolutions for old Pokemon are pointless and stupid and I don't understand why anyone would ever want them unless they are a diehard Little Cup player. There's a good reason why we haven't seen one of these in 4 generations.
From a non-gameplay perspective? They're really cute, and depending on the Pokémon can really help flesh out and visualise its growth cycle. Lapras and Absol, for instance, could benefit from that. And then there's other cases like Kangaskhan where that baby in the pouch is just begging to be its own thing.
(...)
I'd simply argue that it's just a case of most of the examples we've been handed so far not working well, and hence giving the impression that the whole concept is bad. But in reality, Budew and others present a very strong case for the concept to continue.
Adding onto what was already said, they could also be used to maybe teach a Pokemon a move it wouldn't learn normally. Sadly they've only really made baby Pokemon just a smaller, limited version of the Pokemon they evolved from instead of using the opportunity (mostly via Move Tutor) to have them learn something surprising.