A specific league for an "all round bot"? I think that's a brilliant idea but I foresee problems with computation times.
Not necessarily, the fights would just have another random attribute, the map and physics. It doesn't have to take more computation time.
I expect fights not to have a definite win/lose like in db2. Continuing till you hit 95% confidence. But let the rating system take care of it, so a fight is just one fight.
Prehaps a system where, when a new bot reaches a certain rating (possibly above the worst bot in the "top league"), it can be entered into the "top league" and pitted against each one of the bots and then placed (or not placed) in the league accordingly? Unless that is what you were trying to say?
Is one possibility. You could also extend the "top league" with one more bot each time the ranking in the "top league" is rock solid, or even decrease if multiple new strong bots appear.