I would definitely add a group of bottom feeders to spice up the lower part of the tank. Lots of people like to add ghost shrimp, but sometimes bettas decide that they make tasty snacks, so I don't know if I would do it. 4 or 5 panda cory cats would be great. If you manage to find the dwarf or pygmy corys, you could have a school of 8. Corys are adorable! African Dwarf Frogs also do well with bettas, but they can be tricky to feed because they're slow to find food-- especially with all those piggy bettas around.