Personally I would love to see a system like Megaquarium, where you can alter water stats, decorate the tank yourself and select a preferred combination of fishes.
I would imagine such a thing would be possible.
I'm viewing it as a sort of 'advanced' exhibit system. So you still plop down a preset box, and the environment is based on what you put in it, but the options are expanded upon. Ideally yes, it would allow you to add in more than one species up to a maximum capacity, and decorate the tank with pre-placed items such as rocks and plants. Corals are a little trickier, since you'd have to wonder whether they would include them as animals or scenery or something in between.
In terms of water stats, beyond temperature there isn't a whole lot you need to worry about with saltwater. Outside of the extremes, the whole ocean is always a little bit alkaline so pH requirements don't vary hugely for different species, and salinity isn't a big factor either. If they included freshwater fish as well, then certainly, sliders for pH, salinity, and temperature could all be included.