You should be able to "close" a landfill once it fills up.

I, for one, hate having to empty landfills. I think that it would be cool if once a landfill is full there is an option to "close" it. The landfill would remain there for a specified amount of time and would gradually decay over time. Pollution levels would still emit from the decaying landfill until it completely decomposed. That way, you would be able to build and zone on top of it, albeit at the cost of exposing new developments to pollution. It would be a lot of help when retroplanning my cities when they expand!