add sprite frames in g3hs:
just use like nse or unlz-gba to insert 256x128 or larger images and then repoint it. both of these tools repoint automatically. sprite frames will not by default work, so you need to
repoint the animation table.
form changes themselves have been coded by fbi.
different gender sprites has not been coded, and would require an overhaul of a.) the pokemon loading system or b.) the sprite loading system. i personally use different indices for the major gender differences (unfezant, meowstic, etc.)
vivillon would probably be another different indices thing. geographical location would probs fail you ahaha, so i personally suggest like the default or your fav design