I'm actually looking the most forward to see how old Pokémon will get new tools, and how they can use them.
The last generation, fan-favourite-but-generally-useless Politoed went from zero to hero. Dragonite got a spot in the limelight as #1 in the usage stats once, thanks to its new ability. If we go further back, we see Scizor rise to the top spot with the addition of Bullet Punch in Platinum(?), Wobbuffet be banned because of its ability, etc. Pokémon we regard as useless, or worse, mediocre (so nobody even joke about them) can rise to stardom in a generational shift. Some surprise us entirely, going from "on the verge of NU" to "on the verge of Uber" with a single blow. Pokémon we had never imagined to be useful, suddenly find themselves among the top 10 most used. Whether it is a new move, a new ability, or just how the general metagame changes (Amoonguss, Gastrodon), it's always fun to see a "dear old friend" soar from the bottom to the top of the barrel.
And then, there are the fallen stars. Yet, those can still kick ass in lower tiers, biding their time to rise to the top again some later generation...
Likewise, the constant addition of new moves, abilities and changes in availability can make previously boring Pokémon enormously fun to play with in-game. BW2 gave us Riolu from the start. Over the generations, some Pokémon's movepools have figuratively exploded, giving them a plethora of options to use throughout the story. Earlier, you may have been "forced" to use The One Decent Move if you want to use a specific Pokémon in your quest (and it might have learned its last remotely useful move at lvl 36, meaning it won't progress a tiny bit after you've cleared the fifth gym). As the generations progress, you get a lot more options, making each Pokémon more and more interesting to play with. I'd say Beedrill is an excellent example of this. Try to compare its moveset between the
first and
fifth generations.