IMO, there's still plenty of room for small fish. The shark may cause some trouble if you want to add peaceful slow moving fish.
If this was my tank, I'd add two bristlenose plecos, small group of cory catfish, some top swimmers such as dannios or killifish, or maybe some rainbowfish. If you get schooling fish, then you're less likely to overcrowd the tank since they like being in large groups. But be careful of water parameters. The pH might crash if you're not carefully testing on a regular basis. Obviously you don't want to add too many fish at one time. Or it could cause an ammonia spike and make all the fish sick. Just take it slow, observe the tank for signs of stress and test often.