i loved the switch to 1UPT. makes battle seem much more strategic to me.
i agree with the clutter and movement issues. i think there is probably a compromise somewhere. maybe allow stacks of the same unit, or stacks of limited size?
As someone who hasn't played Civ 4 I couldn't imagine going back to infinite stacks of units. I enjoy the 1upt although I could see a switch.
I think they could change it to a standard stack value and have every unit contribute a certain number toward it. For example, say the beginning stack value is 1 and an Archer has a .4 and a Warrior and Spearman each have a .6 stack value. So then you could stack the Archer and either a Warrior, Spearman or Archer but you couldn't stack a Spearman and Warrior.
Afterwards, when the game progresses your stack value would increase but so would the cost to stack newer units. So you might have a stack value of 2 by the time you can build the Swordsman but the Swordsman would have double the cost to stack (1.2) as the Warrior and Spearman.
Then of course there'd be penalties and bonuses applied to stacking via terrain, wonders, buildings, Civs, ect. For example, a jungle tile could apply a 50% penalty to units in the tile while upgrade such as walls, castles, etc. would provide a % stack bonus for units in that city.
While I think it could work in a Civ game, the main problem would be that it would make the game even more complex for new players. For that reason alone, I doubt we'll see it in a Civ game.