I watched my friend play it and it reminded me of the final mission on Tales of Valor, where you are the Germans and have to defend your base and a bit of territory while German AI retreat.
That was my favorite mission and I think it would be cool if Fortress mode was co-op and maybe had an infinite wave option, so you and your friend could see how long you last.
Even in its current state Fortress Mode is better than Stonewall. The new AT guns are awesome, and its cool being able to build building and all the units you use in skirmish.