I've noticed that a vast majority of player you can target in the battle tab are from the top 10 legions in the game. I was wondering if that was just a coincidence or if it was by design.

Probably just because of your level; the higher level you are, the fewer people there are at your level not in those Legions.

It seems when legions are interacting, they tend to show up more on each other's tabs. The higher legions tend to cluster anyway due to ranks, but I definitely think activity affects it. Having 10 Dysonians on battle tab is kind of silly though.

I find it annoying that players you have an NAP with show up in your battle tab at all. Ok, the option to attack should be there for all players, since there will be consequences from breaking an NAP, but it would be a lot better if that was a separate button. Enemies in the battle tab should be enemies.