I re-iterate CtP2's method of one "army" per tile. It doesn't need to be as rigid as CtP; armies could vary in size unit-wise depending on what those units are - standard infantry would take up one "combat tile", a tank column might take up two, with the basic limit being maybe four at the minimum and techs allowing that to be expanded. Maybe also an option to add an extra tile or two with great people (generals). There's a lot that could be done in that way - adding add non-combat units for secondary effects: workers to reduce maintenance or add/improve healing, great persons moving through armies adding a 10 turn morale [bard/artist] or attack [religious/science] or defense [engineer] or whatever bonus. Could also have an army limit and maintenance cost by number of armies, with great generals negating maintenance and army support costs (e.g. making the army free).
I will admit, though, that some of these may not make sense in the "future tech". How do you explain the ability more accurately coordinate an army (e.g. allowing more units in the army) when all the technology that could allow for such a thing already existed? In the past, new doctrines and new technologies for that, but anything after the modern day internet (or, at the least, radio) won't increase that significantly.
The per-tile limit would apply to armies; other unittypes may or may not be limited. In cases where a player stacks military units without combining them into an army, all non-army units in the tile suffer severe collateral damage in the case of a loss (the idea being that all non-defending units were basically "in camp" and not prepared for battle). In cases of attack... probably allow only unit to attack from a given tile per turn. So you could theoretically move a stack of doom around, but only one could attack, and if you're ever attacked back and lose you'll probably lose the entire stack.
Basically, I'd like to see some more innovation in the vein of the unit representation in 4X games, and an "army unit" mechanic seems like underrepresented way to go about that.