Turns out the GBA games have a ton of toolkits to use:
https://www.pokecommunity.com/threads/rom-hacking-tools-pack-all-the-essentials-to-hack-fire-red-and-emerald.527579/
so it's not like it'll really be that hard to make a sprite flip or something.

I mean if you want to talk actual pokemon mechanics, what could be interesting additions?

If we don't just do a Moemon hack, do we make a bunch of Kaomoji to replace the pokemon instead? (´～`) should we just keep the pokemon as is?
Double battles only would be interesting but is there anything else on top we could add to spice it up? Aside from H-Scenes..
Do we even bother making that original?

very many questions to be asked...