A common sense amount of units permitted per tile.
If 1 hex/tile = 10 miles, then obviously one should be able to have more that 1 worker in that tile.
For example, a conversion list like this: 1 tile can hold: 1 Aircraft Carrier & it's fighters OR 2 Cruisers OR 4 Destroyers OR 6 Man-O-Wars OR 8 Frigates OR 10 Caravels OR 12 Triremes.
4 Attack Subs (688 types) could also be in the same tile, because they are below the other ships and wouldn't interfere with their movement.
2 Ohio Class SSBNs could be in the same hex too, but could only fire their nuke missiles when they are in a tile with no ships (or Ice) above them.
A common sense amount of units permitted per tile.
If 1 hex/tile = 10 miles, then obviously one should be able to have more that 1 worker in that tile.
For example, a conversion list like this: 1 tile can hold: 1 Aircraft Carrier & it's fighters OR 2 Cruisers OR 4 Destroyers OR 6 Man-O-Wars OR 8 Frigates OR 10 Caravels OR 12 Triremes.
4 Attack Subs (688 types) could also be in the same tile, because they are below the other ships and wouldn't interfere with their movement.
2 Ohio Class SSBNs could be in the same hex too, but could only fire their nuke missiles when they are in a tile with no ships (or Ice) above them.
For me, about 1UPT or MUPT, there is no debate. We must absolutely have 1APT. (1 Army Per Tile)
The armies would be composed by built units in army slots.
The scale of armies would not matter, because the number of soldiers in them depends on era and spying, wich can be seen both as automatic in Civ (on contrary it would be too boring to manage them) So there would not be X number of soldiers displayed, but only units in army slots just like units in Civ5.
The number of slots would be gained through a global experience tree. (like in a hack & slash) This would represent massive armies like barbarian ones vs less big ones but more organized like the Roman armies. In this expeirnce tree, the easiest ways to improve your armies would be to rise the number of basic slots : this way, you could beat more easily other barbarians. The most refined technics would be harder to get, but would be better than additionnal slots : for example, Discipline would actually boost all your units.
Also, during a battle, each unit would add up to the others in their own ways. For example, mounted units would be good for charges and killing retreats, as well as various tactical purposes. don't be afraid, nothing would be demanded to the player because the battles would ideally unfold automatically like they did in Civ5, a little like a wargame with focused battles put on autosolve.
Don't ask me how to program such a thing though, I think it would be pretty difficult and ask a lot of documentation. For example, if you can put mounted units in your armies, they should give you an advantage if used properly by your generals (more use of them and maybe different kind of them, unless those different kinds are obtained by personnal experience) or if the other side don't have mounted units. In the same way, you could use large shielded units, pikes, swords, armors etc... each bit modifying consistently the outcome of the battles, being mainly preprogrammed within different factors system.
It would not be difficult to play : for example, you just unlocked the light cavalry for your armies. So, you decide to build two Horsemen units and put them in the flank of one of your armies. The next battle, you fight a barbarian unit without horsemen : you win the battle, with less casualties, and a lot more barbarians got killed when they flied : the calculations for this have been complex, but you could sense the effects of your horsemen by just adding them.
Personally, I see a few solutions.
1UPT is a good mechanic, inherently. However, without taking the AI not being able to handle it well into account, the classic Civilization maps cant handle it, either.
Solutions are larger maps, which isn't a very feasible option with the need for a good performance to be taken into account (and it would likely make the AI even worse, if possible!), so we must find another solution.
Such a solution, I think, is smaller hexes. Currently, the average unit can move 2 tiles per turn, a city takes 1 tile no matter the size and a mountain is a single tile.
If one were to split each current tile into X% as large tiles, many options open up. No longer does terrain obstacles need to be so large.
This gives cities the room to grow over time without being overly large (or cumbersome, like Endless Legends). But most importantly of all, this gives far more room to manuever armies, even with the 1UPT system in place. In what is currently one hex, you could fit 7 or 19(?) units. 19 is probably excessive and would lead to problems, so 7 would likely be more viable.
With too large armies, traffic congestion simulator would still be a problem, so production of units could still be kept somewhat close-ish to Civ 5. With seven times as much room, there would be plenty of available space on the map for city expansions, improvements and army manuevering.
If you do not wish to change the tile system, you could use a squad/army based system. Endless Legend does this well. You still get the tactical element of countering opposing troops that you lose with doomstacks, but you keep some of the "the map isn't filled with troops in every tile!" that the current 1UPT gives you.