New ways of connecting levels of multi-story modular buildings
Two ideas I can think of, which are not using "tiles" as such, but similarly flat surfaces:


Technic beams - holes attached to studs of the lower part of the building, and in some holes (which have to be above tiles), pegs/axles for the upper part to fit into.
1x1 "cheese" slopes - and inverted on the upper part (or use regular inverted 33° slopes), which makes the upper part fit in the lower as if in a funnel. (Not very different from using tiles)