How about implementing asteroid belts into some maps. These rocks will have to be destroyed before getting to your opponent. Could possibly make for some interesting choke points and new strategies about sending quick, high-dps frigates first to open weak spots in the asteroid belt before the rest of the fleet arrives. Could also prevent fighter rushing, and will offer slightly different landscape to each map. Space isnt empty.
Space is a lot more empty than you think. Even dense asteroid belts usually have hundreds of miles between rocks, and although this game is as much about realism as Serious Sam is about finesse in gunplay I think that including asteroid belts would make it seem like arbitrary restraints were being forced on the game. It would also get in the way of cool spacesplosions and (given how maneuverable fighters are) make fighter rushes more effective, not less.
I actually think a map with asteroids could be pretty fun, as it would change up strategy a bit.
Remember, this is a video game, not a space simulator 
It would be neat to have some randomly scattered around in the area outside of the deployment zones. I big wall wouldn’t be necessary, but something to make the ships scatter a bit requiring looser formations. That or you have ships with weapons and orders to specifically clear a way through the asteroids for the formation.
Better yet, just stick a big indestructible rock in the middle of the map. As it is now, all ‘maps’ are the same, the only difference is pilots and cost. Sticking an obstruction out there would add strategy to the initial deployment (I’m thinking HoMM here), and make speedy ships worth more, as they could go around the rock faster to get in position to fire while slower ships would struggle.
Putting a few destructible small rocks around would start the pew pew fireworks right away, that might be a good thing? They should blow up right away though, just there for the eye candy.