I think the game lacks replayablity but it could be corrected. I don't play the story mode, only custom games.
First of all, every single game has the same pacing. You struggle at the beginning, then do some game moves to beat the ai at the middle, then just steamroll it at the end game. During end game only thing that holds you back is the lack of durantium for legions (i don't know how the ai gets durantium for its defending legions).
So what would be interesting is some encouragement to keep playing your own custom race. So I suggest that every time you conquer a galaxy, you would get some score to level up your race. So after winning a custom game, you could add to some attributes like construction, science, warfare or whatever for your race. And then these attributes would apply for the next game. Pretty simple, but encouraging.