There are many options. It might be worth looking at
existing issues,
BIPS (although they already propose solutions, they might give you an idea of problems and solutions in Bitcoin) or
wiki (there are some wishlists, too). By the way, Bitcoin Core is missing an HD wallet (see BIP32).
You may also have a look at Namecoin, Ethereum or other altcoins. There is really a lot of possible things you could do. Good luck finding the right one.