I love your arena for the point your at now! Its definitely able to maximize the promotion bonus that you are going to receive, and help slingshot you to D.III. It will probably be one of the better arenas (if not the best arena) in your IV league. It won't be necessary to build more until you know you your promoting to D.III. and even then it not going to be a "necessity".
So to answer your original question:
I'm just looking for what y'all would do next if you had my arena.
My next building project would be 1000-2000 Bleachers (no rush though, because a bigger one might not even be much use until you reach D.III. Just don't go building a ton of Bleacher seats (perhaps a max of 13,000-15000), because you will regret that later when you want lower tier.