Nuclear waste reprocessing
Currently, we have an infinitely increasing nuclear waste when using reactors, and we can't dispose of it, which means it's impossible to build a fully sustainable late-game factory using nuclear.
It would be nice to be able to reprocess it. In RL, "In nuclear reprocessing plants about 96% of spent nuclear fuel is recycled back into uranium-based and mixed-oxide (MOX) fuels." (wiki)
It will solve the issue and could make an interesting production chain.