This sounds like an interesting idea and I admit I've played many games that work like this such Final Fantasy Tactics, Advance Wars, Ogre Tactics, and many others. It does add a whole new dimension to strategy that Civ hasn't had in the past.
But the problem I see is late game with LOTS of units. I typical have over 100 units by the end of most CIV games. At least 3-5 defending each city of which I often have over a dozen, so how does 1 unit per tile work for City Defense?? Also I run around with 2-3 stacks of 20-30 units. The one large stack I found is not that effective cause it's easy to flank an enemy by holding the line with a decent size stack and raze all their cities which another large stack from the flank or sea if no land path is there.
I think the best comparison to my "problem" with this approach in to compare it to Advance Wars. Since in it you capture cities and factories for money to buy/build more units. Well I've notice a common tactic on large maps so to take the Chinese approach for building lots of cheap expendable infantry. This tactic works extremely well as it prevents tanks from advancing since they can't "run over" the infantry. And they must then waste a turn attacking them.
Also in late game with Advance Wars the board because very congested with all the units making it hard to move new units to the front and withdraw damaged units. So you have to leave units on the front line to simply die. I liked the stack in CIV as healthier units would defend first to protect the weak ones. With 1 unit per tile it's gonna be easy to take out the weaker units as well as harder to pull them back. There is really gonna need to be a "swap" places option so on your turn you can have the front line melee guys fall back 1 hex while the reserve guys move into place.
Personally I wish they would do something like Master of Magic. For those who don't know Master of Magic was basically Fantasy version of Civilizations. The combat system though was AWESOME and frankly I wish more games of it's type had done it. What it did was instead of 1v1 number crunch like CIV does it took you into a Turn Based battle kinda like the games I mention above. The great thing about it though was all the units on not only that Tile Appeared but also on the Adjacent Tiles. And where is what I mean.
Say you have setup like so.
D=Stack of Defender Units
A=Stack of Attacker Units
If I remember correctly it used the tile being attacked as the center Tile for determining which tiles were included. What this means is that if either of those Attacker Unit Stacks attack the city then the Defender Units on both sides would get pulled into the battle but also The attacker could use both stacks in the attack. This was rather important as there was a limit on how many units you could fit on one space but this combat setup allowed you to pull in more units for rather large battles.
The company that made Master of Magic also made Master of Orion Series. The company got bought out so a sequel never got made . There was a game made that was suppose to be a "spiritual" successor but it turned out more to be a copy of Heroes of Might and Magic rather then a true Master of Magic sequel.
I recommend looking up how the combat system worked in Master of Magic as I think it would actually be a great way to handle combat in Civ5. To many games now seem to want to do RTS Combat in TBS games which I don't really like as I buy a RTS to play and RTS and when I buy an TBS game it's cause I want to play a TBS not a watered down campaign mode RTS game. This way the game maintains it's TBS nature and gives the player much more detail on the battle level without blowing things out of proportion by attacking 3-4 Hexes away. I mean if archers can attack 2 spaces away then it seems only reasonable that late game stuff can attack even further. In previous games when the cities had to be at least 5 squares apart to prevent overlap the ability to shoot 3-4 hexes away seems unrealistic as your bombarding the from one city to the other. Which is kinda what happen with air planes in Civ4 once you got to that Tech point. But to see that kind of thing with artillery seems potentially excessive.
To be honest the best concept of combat was Civ CTP. Ok it was incredibly simplistic but much more personal than any Civ.
I think that how it should have been done, but heavily improved of course.
Limited units per tile could be a good idea, but it needs to be more than 1 unit per tile.
Having ~5 land units per tile (not counting units loaded on transports) and having them fight as a cohesive unit would add a lot to the game.