A ground combat add on for the campaign would be good - firstly you have to defeat the space fleet protecting the planet, then subdue the ground forces.
Also, I have always thought that having a time based scenario would be fun. For example, holding off the attacking forces for X amount of time, after which the reinforcements arrive.
This could be extended to escort duties - protecting Ship X while it tries to make a hasty exit. That would go well in the campaign where you protect transporters (the transport ships in the Praetorian mod would be a great fit in this scenario) craft ferrying supplies to a planet or a delivery of a new Dreadnought to a far flung outpost. It could be either time based, or have a goal of reaching a certain point on the map to win. This adds another dimension to the (very fun) but sometimes one dimensional goal of battering the other side until they have nothing left.
honestly, i would prefer if GSB stayed in the 2d isometric view. this may seem contradicting as i make 3d models, but doing it in 2d clears up alot of problems. the main one being that its easy to mod.
it also means that anyone can play the game, regardless of how sucky their current gaming station is (my old computer was basicly a toaster hooked up to a fat screen and a calculator as keyboard and it plays gsb without any issues)
and since Cliff is an indie developer, he needs as many customers as possible to pay the bills. it wouldnt make sense to limit those to people with only high end systems.
2d means that he can make an amazing looking game with only a third of the computer power.