This is a heavy one, but crucial to keeping the workflow going within the same environment. If the beams have too many holes for pipes, the floor might collapse. If there’s a deep pool on the top of the building, the ceiling might collapse.
If a designer wants to make a futuristic, fancy wall-attached toilet, I want to be able to define the contact surface and where the bolts are holding it. It needs to be sturdy enough not to break itself when a person sits on it.
Of course, how the toilet is attached to the wall should have little to no influence on floor’s stability. The software would recognize those small-scale loads and use approximations. If that’s too big of a problem, we should be able to exclude specific elements from calculations or approximate them ourselves.
Then there’s environmental effects. Those are included in some standards so that might make it simpler on simulations. For example ISO 4354 covers winds, ISO 3010 covers earthquakes, etc.
Edit: I don’t know how performance-efficient will this end up being. You might have to charge for it separately or allow for the user’s PC to do the calculations. If latter is the case, it would be good if a simulation could use specific CPU cores so the user can still use their web browser in the meantime at least. Or for the simulation to be split up between connected PCs, but this would probably require some wizard to pull off properly.