For a 70 g with good filtration that is not overstocked at all. The problem here are your kuhli loaches and tetras. Very tasty snacks for most "oddballs". synodontis are always cool and usually dont go after small fish if fed. Butterflys will stay at the top, but they are still not to be trusted with your danios...if they venture near the top they will be a target. There are some beautiful varieties of killifish(check out aquabid) that are not readily available at stores. leaffish probably wont be much of a nuisance.
If you move the loaches, shrimp and danios to their own smaller tank, the only limiting factor would be the tank itself. It would open you up to everything from larger cichlids to predatory gobies to bichirs and eels. There are also nice substitutes for those fish if you wanted to change, weather loaches or ropefish, larger danios, or bamboo shrimp will stay large enough to allow more variety of options. Just my 2 cents