I think a great feature would be to add freshwater fish, maybe have an option when placing a tank to fill it with FW/SW. You could have your standard small FW fish like tetras, angelfish, and barbs, plus bigger fish like arowana, FW stingray, electric eel, and knife fish. Also any oddballs in between.

It may also be interesting to have breeding occur if water quality and conditions are perfectly met (100%+) but rarely occurring depending on the species. Either laying a few eggs that hatch in x amount of days or just juveniles appear with a notification above the tank.

...Also, can someone add a sawfish please?? :D