Well, which is it, a 40 or a 50, and then what kind of dimentions are we talking about? Would it be a long 4 foot tank, or a tall 3 foot tank?
I've been wanting to set up an african tank in a 40 gal. long or breeder size tank for a while now. I'd plant it with tons of anubias and maybe some crypts and have one or two senegal birchirs, a few African butterfly fish, and some marbled or leopard ctenopomas. If I thought the bircirs wouldn't eat them, a school of congo tetras might look nice as well.
That size tank is also nice for a small african cichlid community. You can have a bunch of Mbuna, a group of tropheus, or a whole tanganyikan setup with shellies, birchiardi, julies, and comprisseps, and maybe even a few petricola syno cats.
I'f you're more into South American fish, there are endless options. I personally really like firemouths, perhaps with a bunch of some kind of barb. You could have a nice collection of some of the smaller plecos as long as you don't mind cleaning up after them. There are a lot of species that only get to about 6-8 inches. You'd want a long or breeder sized tank for those.
You have a whole bunch of options. Nothing really large, but there are lots of possibilities anyway. What sort of fish do you really like? What havn't you ever done before that you'd like to try? What look and feel are you going for, and how much effort do you want to put into maintaining the tank and its inhabitants?