Probably been suggested before and I know Gateway traps exist, but I think having wall portals would be interesting. The idea is that they only appear along a wall and work both ways instead of the Gateway traps unidirectional flow. Wall portals could work in just one room or perhaps joining two different rooms. And speaking of Gateway traps, what if there was a different color variation that sent you to an adjacent floor?
Might be quite difficult to balance. As a ranged character for example, unless enemies can punch “through” the portal, it hard shuts down any melee enemy and even e.g. prison guards after they do their one shot.