What old features need a desperate comeback? From Yellow, we need to see certain pokemon in your party tail you where you go. From Gold/Silver/Crystal, I believe the puzzles we had to do to unlock unknown were fun, the Pokeradio and it's channels, bug catching zone, specific pokemon battlegrounds (Eevee house), multi-region gameplay, and Red, the ultimate trainer, waiting for you at the top of the mountain. From Ruby/Saphire/Emerald, underwater areas, berry blender (so much better than Poffin baking), and creative ways to use the bike. Personally, the ones I believe are in most need are the bug catching zone and the multi-region gameplay.
Man, what we really need is a game that just spawns throughtout every series. You start in Gold/Silver/Crystal area, move to Red/Blue/Yellow area and fight the Elite 4 with Drake having a level 42 Dragonair as his highest level pokemon. Progress to Ruby/Saphire/Emerald territory, fighting the Elite 4 with Steven having a level 60 Metagross as his MVP. Progress finally into Sinnoh, destroy the Elite 4 with Cynthia having a level 80 Garchomp as her most powerful pokemon. Then, return to Gold/Silver/Crystal and face Red at the top of Silver Mountain, and he has a level 85 Pikachu, a level 87 Venasaur/ Blastoise/ Charizard, a level 85 Snorlax, and a level 90 Tyranitar. Then there's online play and GTS to occupy you beyond that. That would possibly be the most perfect game ever.
Man, what we really need is a game that just spawns throughtout every series. You start in Gold/Silver/Crystal area, move to Red/Blue/Yellow area and fight the Elite 4 with Drake having a level 42 Dragonair as his highest level pokemon. Progress to Ruby/Saphire/Emerald territory, fighting the Elite 4 with Steven having a level 60 Metagross as his MVP. Progress finally into Sinnoh, destroy the Elite 4 with Cynthia having a level 80 Garchomp as her most powerful pokemon. Then, return to Gold/Silver/Crystal and face Red at the top of Silver Mountain, and he has a level 85 Pikachu, a level 87 Venasaur/ Blastoise/ Charizard, a level 85 Snorlax, and a level 90 Tyranitar. Then there's online play and GTS to occupy you beyond that. That would possibly be the most perfect game ever.