If you wanted to go for something a bit more exciting and colourful with a bit of personality you could try some of the larger africans.. Giraffes, dolphins and comp's commonly get to 15-20cm's (goto butterfly creek ), most other africans max out at 15 cm's with some haps getting larger..
Or go for frontosa.. But thats big money and your tank isn't big enough for a grown colony (takes years for them to get that big..)
Really depends the colours you want, how much you have to spend, if you want it planted, what types of fish you have access too etc etc etc..