1 UPT is not neccessarily a bad mechanic
Low movement allowance is not neccessarily a bad mechanic
Not being able to stack with “foreign” units is not a bad mechanic
combining all three is like...what were you thinking?
IMO, every tile should be able to hold the following:
- 1 military unit
- 1 support unit
- 1 religious unit
- 1 Great General or Great Admiral
- Unlimited civilian units (it’s not like you can have a “builder doom stack”)
- Unlimited non-military Great People
Also, foreign units should be able to share a tile unless one is presently capable of attacking another without declaring a new war (i.e. military units and any other units they’re at war with, or any religious units they can condemn). If their attackability changes (e.g. if war is declared) any non-military units stacked with military units that can attack them get pushed to a neighboring tile (it would make more sense for them to be auto-attacked, but that would probably allow too much cheese).
It would mean you can’t block other civs’ settlers, but that’s cheesy anyway (if not being able to do that is a problem, the solution is to make the AI forward-settle less). It would also mean you couldn’t use non-military units to wall off city states from their attackers (useful if an ally decides to conquer your vassal), but that definitely needs a better solution anyway, e.g., suzerainty being treated as a defensive pact and the AI not attacking city states unless it is willing and able to declare war on their suzerain (this is actually one of the main things I hope they fix in Civ VII—or a Civ VI expansion if we get another).