I think there should be battles against federation fleets in single player because you have to fight against fleets of your own race with the other races, too. Another cool feature would be race specific battles that you can only fight if you have unlocked a certain race.
agreed 
+1 to this for the most part.
In the interim, however, hop onto challenge mode; there’s some interesting federation fleets around there. Also have a look at the Scenario Editing Tips thread. This demonstrates how with a little text-editing effort on your part, you can create whatever scenarios you’d like.