There is gravity already, players, vehicles, animals are not floating in the sky, even wood and stones are falling when cut. What could be a nice visual improvement is to have some support rules for placing/removing blocks, checking for adjacent blocks. It could depend on the building material, to be able to make nice roofs, overhangs, but no floating roads or dirt.
I like this idea because going through a realistically generated world and seeing floating structure kind of kills the immersion and just looks ugly, but I think it would be a complex thing to change and do right.
I my opinion it would be too complex and it wouldn't be Eco anymore :x I mean.. it's not even possible to die.
Pancake storages and one-tile flying bridges drive me crazy!
Interesting support system in the game Star Rupture - need a ground support every four blocks, but given that the property system is built on claims (5x5 plots), and considering the fact that the most popular warehouse is also 5x5x5, I can't imagine how such a system could be usefully implemented. Also, a fixed distance between supports would hinder player creativity, and all buildings over one floor would become uniformly aligned with the support grid.