Touching up on a small issue I couldn't fit into the OP: Content inflation for single Pokémon.
Consider any Pokémon introduced in Gen. I that was not a Normal type, Dragonite, Gengar or Nidosomething. In its original form, it would have a pretty abysmal movepool, could not hold items, and did not have any Abilities. All in all, quite boring by today's standards.
Gen. II expanded its movepool to passable levels, balanced its Special stats, and gave it the option to hold items, effectively making it a lot more diverse to play with.
Come Gen. III, it got an Ability, maybe even two, more items to hold, and more moves. Tutors for stuff not called Headbutt or Elemental Beams enter the scene. Breeding moves are expanded upon, and Heart Scales mean that fully evolved Pokémon can have "hidden moves" theoretically learnable at lvl. 1, but in reality only available via relearning.
In Gen. IV, the great Physical/Special split gave our theoretical Pokémon a lot of more viable move types to play with, allowing for a more diverse movepool. It might have gotten access to a new Ability, and maybe also an evolution. Game mechanics as we know them today are mostly finalized at this time.
By Gen. V, the Pokémon is not changed as much from the previous generation. It might have gotten a Hidden Ability, which is to say a regular Ability with a more inconvenient way of acquisition. New moves are somewhat limited in number and distribution. Tutors' learn lists are nearly filled up with already-existing moves.
Gen. VI might add Mega Evolution to our theory-mon, but little in the way of new moves. Most new moves have a rather limited distribution, and the new Abilities even more so. Apart from through Mega Evolution, only a tiny handful of old Pokémon gain access to one of the new Abilities (the exceptions only being Competitive and Protean, given to seven and one old Pokémon, respectively). A handful of Pokémon and moves see some adjustments to their stats, which otherwise have been unchanged since GSC or longer.
The trend is that Pokémon gradually gain new content for them every generation, usually in the way of new moves or new Abilities. However, from a design standpoint there's only so many moves you can give a Pokémon before they are saturated, so to say. You don't need a new move every third level, for instance, and if every Pokémon learns every move, diversity is lost. Give Drill Peck to Pidgeot and Hurricane to Fearow, then what is left to distinguish them from another*? Movepools have ballooned every generation now, but the pace is going slower as we reach a point where (gasp!) most Pokémon already have every move they could wish for. The addition of several abilities makes opportunities for several play styles, but then again there are other factors judging whether a Pokémon can play that style good enough, for instance stats (and movepool, given that movepools are kept limited to uphold diversity). Some Pokémon will always do the same job much better, meaning that each Pokémon will face stiff competition for every niche it tries to occupy, and the worse ones will be left without a niche at all.
Keep in mind that niches aren't solely competitive either, there are also niches for general notability. For instance, Lotad's line was long the only Pokémon that could recover HP in rain thanks to Rain Dish. Then Gen. V arrived, and eight more Pokémon gained the ability. Plusle and Minun had this thing going with Plus and Minus. Well, now there are four other evolutionary families having the option. Pain Split was once Misdreavus' signature move, now there are almost a hundred Pokémon capable of learning it.
Spread the options too widely, and traits that once made a Pokémon unique become commonplace and the uniquety - the Pokémon's very identity - disappears.
Keep in mind that Abilities usually give a Pokémon way more of an identity than a movepool does. That is, signature moves add flavour too, but not nearly as much as a (unique?) ability does. If a Pokémon has too many possible Abilities, what identity does it have? Give it Guts/Sturdy/Levitate, and it'd be hard to tell what the Pokémon actually
does. A Pokémon with only Arena Trap, for instance, has it fairly simple: Just from that alone, you can tell that it traps its enemies, forcing them to battle as there is no escape (unless you can fly). If the 'mon has Telepathy, you can picture it communicating voicelessly with its teammates. A Pokémon with Strong Jaw bites a lot. Even though its movepool might not necessarily have as many biting moves available, players will immediately associate it with biting (Hippowdon, for instance, learns more biting moves than Mega Sharpedo, even though the latter clearly is easier to associate with biting attacks). A good Ability is one that captures the essence of the Pokémon. Give a 'mon too many abilities, most of which are shared with other, completely different Pokémon, and that essence is going to be awfully diluted.
However, and this is the big key, part of the fun experience with playing Pokémon, and a
huge marketing draw for the series, is seeing how old Pokémon is given new content to play with for every game: More Abilities. Egg Moves. Tutor moves. Updated learnsets, so the Pokémon is more fun to play through the game with than it was in the last game it featured (as illustrated perfectly by many ORAS Pokémon). We do not only crave new content and new Pokémon, we crave new content for old Pokémon. We want Hi Jump Kick for Breloom. Heavy Slam for Metagross. A physical Ice move with 100+ base power for Kyurem-B. Tinted Lens for Flygon, Flare Blitz for Flareon, Aeroblast for Aerodactyl, all those old memes. And we'll get mightily disappointed the day GameFreak creates tons of new moves and Abilities, yet not distributing them among existing Pokémon. Remember the backlash for BW, when old Pokémon weren't available in the main story? Fans
want their old favourites to get some attention too.
In other words:
- To keep variation and uniquety alive, GameFreak has to restrict the amount of Pokémon they spread their new moves and abilities across (in short, preventing every Pokémon from being capable of doing every job - which would lead right into that famous The Incredibles quote**).
- To keep movepools and Abilities within reasonable limits for a game - and to give each Pokémon a sort of distinct identity - a Pokémon should not have too many moves or Abilities available to it.
- Movepools and Abilities have begun to reach their practical limits, if not this generation then within two or three. Content is very rarely removed from a Pokémon.
- Fans still want more, more, more, and not adding so would potentially hurt sales.
So, all in all, GameFreak is at a bit of a catch-22: They can add more content to most old Pokémon, and watch them struggle to have any sort of notability among the hundreds and hundreds of available species (additionally making it harder and harder for new players to pick things up), or they can make less new content for every Pokémon and hurt sales. It's a bit of a dilemma, I'd say.
*"Pidgeot can Mega Evolve, you dolt!" you might be shouting at your computer screen. True enough, the addition of Megas with a new playstyle gives many Pokémon an entirely new breath of life. Mega Evolution might help some Pokémon out of the bog of unremarkability. It is a once-per-battle limited tool, true enough, but average players go through hundreds of in-game battles, all the while rotating and changing their team, so they can try out every Mega they come across and use it to their heart's content without worrying if it is the optimal choice for that team composition.
**"When everybody is super - no one will be". Yes, I had to look it up too.
