First you'll need to look into fishless cycling the tank (look at the link in my signature). Then there are hundreds of possibilities.
You said something unusual... well killifish are somewhat unusual. I think a pair of badis would work. A single south american leaf fish might though I have limmited knowledge of their requirements. There are puffers that would work though I've never kept any. Shell-dwelling cichlids though, again, I've never kept any and am not much of a cichlid fan...
In fact, I'm more a gourami person - you could keep some honey gouramies or a pair of dwarf gouramies. Croaking and sparkling gouramies would also work. They croak so are somewhat unusual... Licorice gouramies if you are prepaired to research them, may also be possible though more fragile.
Then there are all the typical combinations of small tetras - black phantoms are the best - 5 of them would be beautiful if you planted the tank... the males will display to one another. 5 harlequin rasboras in a similar setting would work and there are various similarly-sized rasboras that are rarer but just as suitable. Golden dwarf barbs are great too.
A few otos would be nice. Try getting them to breed - that might be fun in a planted tank.
You could probably get 4 cories though they are somewhat active for so small a tank - pygmy/dwarf cories would work though and you could probably put 6 in there. They aren't bottom-dwelling so much either - so they'd be hanging around the middle layers in plain view - so cute!
I must mention a betta... with a couple of otos and maybe some rasboras.
And of course livebearers - guppies or platies would be nice and they'd breed for you too. Alternatively - get 8 mosquito fish or some endler's livebearers. Don't mix guppies with the latter though and be prepaired for lots and lots of fry (they don't eat their fry as much as other livebearers tend to)!
As I said, there are limitless options... what are you looking for? Species tank or community? Active, peaceful, colorful? Do you want to breed the fish? What sort of set-up? Planted? etc...