I find that the current spot, (bottom right), is awkward because the user has to glance over frequently to see if it’s usable. Placing it in the center would be ideal.
The buttons in the middle (Quest, Shop, Spectate, etc.) are only available in the lobby, not during gameplay. Therefore, I could position the ability UI in the center once the game starts. However, where should it be located in the lobby?
I believe that the bottom right spot is perfectly fine.
You state that the player may have some slight annoyance glancing at the ability, but if it’s only there during the lobby, I think it’s fine!
I am assuming that there isn’t any “gameplay” in the lobby, so just glancing won’t hurt.
You definitely should put the ability UI in the center bottom while in game though.
If you are still worried about not knowing if your ability is usable in the lobby, some good SFX can easily fix that problem. A lot of games with abilities use SFX to indicate a recharged ability, an enemy windup, etc.
Aside from your problem though, the UI looks SUPER good! Nice and sleak, my type of style!