Hmm new tech systems and other new game play features are usually what interests me most. Anything that's different from the original tech upgrading/slicing makes a mod a lot more interesting.
Some small events during a GC are also an important feature in my opinion, things like that keep the player playing the mod.
It would be also interesting to know what your "unique economy system" looks like.
A new economy system is something I've been trying to create for some time now, but I'm lacking a good idea