I and a couple other players discussed that as a possibility. Stardock hasn't given any official news about future DLC.
In my opinion, an improved combat system would benefit Gal Civ IV the most.
The other aspects of the 4x genre, exploration, expansion, and exploitation are very well fleshed out. The ship designer is incredible, and there is a lot of variety in ship modules.
The way they combat system is built doesn't make use of all of that variety though. It actually encourages the player to build homogeneous fleets with maybe one or two supporting ships. This is because ship speed is one of the most important stats in the battle. Faster (smaller) ships often get into combat and destroyed before large ships reach the fight. This splits your damage and defenses leading to a much worse outcome in the battle.