Hmm, oldschool black and white gameboy huh? Hmm, I really can't give out any suggestions, well I can, but people aren't going to like what I say. To be honest, I tried out pokemon for gameboy, and I gotta say it ain't half bad.
if you like Diablo for Pc, then you'll like Lord of the Rings the two towers & Return of the king, they both have very similar play like Diablo, they even have pretty much the same mana and health bars in the same bottom location. It's pretty fun, each characters quest is different, they both offer 5 playable characters in the game.
For turn based strategy try Advance Wars 1 & 2, they are very addicting and good for those long bus rides home or whatever.
Oh hey, also you might want to check out the upcomming Harvest moon: Friends of Minera, if you played original harvest moon for the snes you'll know how this one player. it offers farming, cooking, dating, fishing, all in one. Or you could just do that in real life. lol
I don't know to many GBA games... heh.