I would love to see some form of client-side integration (API) that would allow retrieval of certain information (e.g. position, ship data, trade prices) as well as manipulation of certain things (e.g. setting navigation routes). I wouldn't want to see anything that fundamentally changes the game, though. That would include adding assets, ship types, etc. I'm all for enhancing gameplay, not changing it.
Here are a couple of other thoughts:
I made a comment a while ago with regard to the never-ending open vs solo debate. It was in discussing the idea of splitting the BGS for the different modes. One of the problems that I pointed out was that FD are building a narrative of sorts along with the gameplay. The events and actions that are announced in the newsletters and galnet affect all modes equally. If the BGS was split, that consistency would soon disappear.
The same would likely apply if mods indeed became possible and got out of hand. Could you imagine FD planning the careful steps leading towards the eventual discovery of Thargoids only to have someone release the Super Awesome Thargoid Mod? The storyline would become completely irrelevant.
Support would also become troublesome. We are playing a game that is under constant development and FD works with the community to iron out bugs as they come up. Throwing random modifications into the mix could make this process significantly harder.
There's also the argument that the mods would not be supported in all environments. Unfortunately, that same argument applies to many third-party apps. So I won't focus on that one too much. [uhh]
I won't even touch the potential for cheats.
Overall, though, I would prefer that FD's main focus remained on fixing current bugs and plowing ahead with future content. I would rather land on planets with atmosphere (I'm sure the XB1 players would just love to land on planets) than be able to change the voice of the ship feedback or colour of the HUD.