Pretend you have a... Kecleon whose main move is Return. You just forget Return right before the battle and teach it Shadow Ball to one-shot Shedinja, and relearn Return in its place after the battle.
Murkrow evolves and she gains Weavile, Spiritomb, and Absol, none of which existed when she was created. I'm fine with Drake having Hydreigon as long as it doesn't show up the first time you battle him. As long as they leave the iconic pokemon like Tate/Liza's Solrock/Lunatone and don't replace them with Reuniclus/Gothitelle
Gym Leaders could also use more screen time in my opinion.
Personally, I hope that by the time they make the RSE remakes they've also brought in a Banette evo, 'cause not only does it seriously need to happen but Phoebe would kick ass with one.
Aren't Wattson, Norman and Wallace the only Gym Leaders that appear outside of their gym, whereas Steven is the only Elite 4 member who does? We definitely need to see more of Hoenn's trainers.
They could easily add Flannery to the Team Magma plot at Mount Chimney, add Tate & Liza to the Mossdeep City (if they keep it from Emerald; replacing Steven possibly), and put Phoebe in Mt. Pyre. I'm sure they'll be able to add others elsewhere as well.
Yeah, but doing that could be a risky move, nostalgia fans wouldn't be happy, and I can't see them altering the storyline at all.
But, it is a nice idea.
@Joshawott: Yeah, that would be a good idea. You could even add in a Sevii Island-esque sub-plot too/instead with the potential for greater leader/E4 involvement. In fact, I always found it weird too that the town gym leaders are so laissez-faire when it comes to crime & such in their area; I still find it particularly jarring that they're so hands on in Unova. That doesn't make it a bad thing mind (quite the opposite), just different.