I think it would be super awesome to have fish tanks, and even cooler if we could customize them. I have a couple ideas and I'm working on an art concept for them as well, but I'm pretty slow so it's not finished as of yet.
For customization, it'd be fun to be able to buy individual fish or even maybe make a mini game out of it where you have to going fishing to catch them. There could be different tank shapes, like globes or rectangles, and maybe even have different pieces like fish tank coffee tables or fish tank walls. Maybe even have different plants, lights, and pebbles that can be placed into them to customize even further.
Even without customization, preset animated fish tank items (maybe craftable?) would be a super fun idea!