Jenaelha
fields of Elanor
1) Split stacks into smaller ones for movement, converge at end of turn.
2) Wouldn't hamper stack effectiveness much since the benefits of a larger stack far outweigh a couple units going stray.
3) Similar to the first one. If needed, stay in separate stacks until next to a city.
This is, of course, ignoring the fact that all three of those options would severely hinder the AI. I said it before, and I will say it again; Civilization IV is built to use stacks of units. Trying to break stacks up will only cause more trouble unless you completely overhaul the game engine. Unless you design a game so only a single unit can occupy a single space, these mechanics can not be changed - at least not in a convenient way..
Well what about exponentially increasing the cost of maintaining units beyond a certain GNP ratio? In fact, how does the AI avoid sucubbming to this crippling burden with his massive stacks now? Wouldn't this be a cost/benefit analysis an AI could make that might curb his enthusiasm for massive, redundant stacks?
I'm not clear on why these options would severely hinder the AI, unless you mean the AI wouldn't figure out a workaround while a human would?