Yeah, I'm not going to add a level cap, it seems too arbitrary. If you use Wonder Traded Pokémon you'll already have to keep them below the badge limits so they'll obey you (although I've only ever had to worry about the level 30 limit before the 4th badge). My goal is to fix the complaints with vanilla GSC, and a common one is that the levels are too low, even in HGSS.
Breeding legendaries hadn't occurred to me.
Maybe I'll include it, for non-mythical Pokémon (so just the trios), and with some extra requirement (like a certain held item that's hard to find). Actually, that could be pretty cool. Thoughts?
I would love if people use my hack for multiplayer trading and battling, but it would have to be between two copies of Polished Crystal. By now I'm sure that trading with GSC is broken, let alone RBY. Some of the Pokémon shifted index numbers, lots of moves have been replaced, and I completely
reordered the items to have a neat arrangement. (Might do the same with moves, since backwards compatibility doesn't matter.)
Right now I'm working on some of the prerequisites for fixing the walking-sprite glitches. For the TMs, I might make them infinite-use for 1.0.1 and ignore the fact that you can keep buying them until later, since it's easy to just not do that.
The battle mechanics
will all get upgraded, eventually. Those kinds of changes just take a lot of testing to make sure nothing broke, and the randomness of some of them has to be verified.