An "easy to implement" note to remove the obvious ship interiors, something like new mechanics with UI or interactions with the BGS.
My first feature would be stolen from Eve Online's Project Discovery it would consist of a new module that acts as a powerful telescope to look at systems in a medium range, 10-30LY.
You wouldn't see planets, but you could do Astrometry (see the wobble of the star as planets orbit around), or Transit detection (see the star light dim as planets go in front).
The data could be taken from real dataset that need human analysis (if there's any left), and players rewarded with both a credit vouchers and a way to look for potential biological signals in surrounding systems.
Another feature that could be easy (but require a lot of balancing or limits) would be to allow players to create Missions.
For example, with colonization, you could give credits to players who participate in transporting resources, you could pay players to bring you engineering resources, you could create escorts missions to protect you from gankers, etc...
What kind of feature would you like to see?