DC's top - tier characters already appear together in DC Universe Online and the newly released LEGO Batman 2: DC Super Heroes, and will reteam next year in Injustice: Gods Among Us.
Also, as I said when I rebalanced Puzzle Fighter, we already know what the game felt like with the previous top tiercharacters, and it was fun, so it's better to balance the game around that power level than a new, lower power level.