Ah, I always understood "doom stack" to mean a stack that is unable to be defended against due to its size. My apologies. In that case, if the problem is one of snowballing, I feel that's a problem with civ as a whole. Without a system like Rhye's and Fall's stability, there is no reason not to be constantly expanding, be it your military, your territory, etc. Even if stacks were limited to 10, that'd just mean the arms race will be one of spamming as many 10 unit stacks as possible and throwing them into enemy cities fast enough that you never have a traffic jam. That said, if we really wanted to limit stack sizes, a few civ 4 mods add a really interesting rule to the game's existing supply rule. Much like how civs pay 1 extra gold for every unit outside their territory beyond X, with X being determined by the civ's number of cities, some mods make civs pay 1 extra gold for every unit in a stack beyond Y, with Y being determined by the civ's number of cities, preventing giant stacks without impeding decision making. As for limiting forms of snowballing like arms races, I don't expect a single Civ game to ever be released where it's not optimum. It was optimum in 1, it was optimum in 2, and 3, and 4, 5, and 6, and I'd be surprised if arms racing wasn't optimum in 7.