The lack of variety is obviously due to imbalance (there is a lot of claims about its because characters like Poe, phasma, sid, and Lumi are easy to get for F2P but they are also used just as much as whales so it's obviously NOT the case entirely). Poe and FOTP are in the same class as the leader issues. On my server there are 6 characters in top 100 as leader. 5 of the top 100 aren't sid or phasma and none of those are above 60!
I always feel for programmers who have to try and figure it out because perfect imbalance is Impossible. But I have to think the goal is either all characters are at least somewhat used and viable or a certain % are viable at top. I definitely don't buy that only about 10 characters are intended to be able to compete.
If you totally think everything is fine as is, I invite you to try a team of only characters you'd assume are intended to be high level (say like mace) that aren't used and see how often you can win