Honestly, you have so many options it's hard to know where to start. For me, I typically like to set up themed tanks around a region. The places I've themed my tanks after most often are areas in SE Asia and S America. For a SE Asian tank, I'd typically get some rasboras, gouramis, and loaches. For a S American tank, I'd typically get some tetras, dwarf cichlids, and catfish. It's totally up to you.