Emerald source is almost useless when it comes to any hacking use because of disassembly existing, again. More beta sprites and music would be nice though (kinda doubt placeholder sprites will be a thing this time around since this is third version). Unfortunately, I think we're about done with early Pokemon revelations for the most part but I could be wrong.
Everything else might have something useful though.
EDIT: Oh hey, interesting that they used spreadsheets to localize. Reminds me of a lot of stuff I've done before.