No buffs, 2 hour total format (90 minutes + 30 minute break). Each round, the top 3 most overused hero is eliminated in the next tournament and this process is cumulative. I counted 36 heroes based on the character compendium, so by the 12th round there should be exactly only 3 heroes left that can be used. There should be some rather interesting team halfway through such a tournament.
Alternatively this can be done on the individual basis. Pick your 3 heroes and you’re locked into those 3 heroes for the rest of that round. At the next round those 3 heroes are eliminated from your roster for the next round.