OK, I obviously picked definite-largely because I feel that the current combat system is too time consuming, and is merely an adjunct to the whole 'micromanagement' problem!!
That said, any stack combat system would DEMAND stack limits, in order to prevent rampant abuse of the system. Stack limits would depend on the terrain in which the units end up in!
Anyway, my preferred options for a 'stack combat system' would be as follows:
(a) Turn-based, tactical miniscreen: This would be the ultimate system for me, where you can move your units and attack other units in a seperate screen during the 'combat phase' of the turn. This would add a massive new tactical element to the game. I realise, however, that a lot of people don't like this idea (largely based on the CtP experience, which wasn't nearly good enough IMHO) but is what I would have if 'my will was law'

!
(b) This option would work in a similar fashion to civ3 combat. Basically everyone would have their move, and any enemy units which ended up in the same tile at the end of this phase would be required to either 'retreat' or 'slug it out'. At this point each unit in the stack would automatically 'have a shot' at a target in the 'opposing' stack-and vice versa-until the target is killed or one or the other unit retreats. If either of the latter happens, that unit will move to attack another available target. At any time, either side can do a 'stack retreat' which will essentially yield the tile to the other side. Which enemy a unit targets could either be automatically selected by the computer or selected by the player prior to combat starting. The similarity to civ3 is that you would see the individual animations for the units that are involved in conflict.
Interesting ideas connected with (b) is that a unit might be attacking one enemy, whilst fending off attacks from several other enemies. In this case, it would get to defend, but get no corresponding counterattack, against the other attackers. In addition, such a unit would probably get a penalty to its DS for every unit, after the first, that it is defending against. This would make numerical advantage much more important than is currently the case.
In addition, units might have an 'attack rate', 'range' and 'firepower' score, which determines how many times they can attack each 'phase' (or how many different enemies they can attack), how many free shots they can get on an enemy (ranged units only), and how much damage they can potentially dish out on a successful attack.
OK, here is how I sought of see option (b) working in the game:
Player A has a stack of 5 units (3 Med Infs and 2 longbowmen-LBMs) which are defending a pass through the mountains. Player B has 6 units (the maximum for this kind of tile) (2 Knights, 2 Med Infs and 2 LBM's) which he moves, on his turn, to occupy Player A's tile. When all movement is done, the two stacks 'slug it out'.
First, each sides LBM's (with a range of 1) get their 'free attack' against units in the opposing stacks. Then, the knights and 1 Med Inf take on a enemy Med Inf each, with the final med inf going 1 on 1 with the final enemy unit-whilst the LBM's of each side continue to lend 'supporting fire'.
On the screen, we see a knight take a swing at its target, and the targets counterattack, followed by the next knights attack and so on. In addition, we see 1 knight and 1 Med Inf get its attack, but with no corresponding counterattack. Also, the two 'defending' Med Infs are at a penalty to their DS (and possibly morale) due to being 'outnumbered'.
This is repeated each 'pulse' until either ALL units of one side are defeated, or one or both sides have 'yielded the field.
To continue the example, though, lets say that two of the knights defeat the enemy med inf, the controlling player may choose to 'retreat' one or both of them or, if he doesn't, then they will automatically move on to any available targets-in this case perhaps 1 knight might target an enemy LBM, and the other might go and help out against any remaining, and possibly weakened, Med infantry.
Now, I know it might SOUND confusing, but please remember that much of what I described will be worked out by the computer, based on its perception of victory odds-with perhaps an element of randomness thrown in.
Lastly, within option (a) and (b), there is still the option of building and loading armies. These armies would represent a greater level of organisation than a mere stack, and would recieve bonuses accordingly. One such bonus might be that each unit in an army would recieve a % increase-in one of its stats-equal to the unit which is strongest in that stat.
For example, if you have a knight and a pikeman in an army (and the Knight has an AS: 50, DS: 15; and the pikeman has AS: 20 and DS:60) then the Pikeman would get an AS bonus of, say, +5 (10% of Knights AS) and the Knight would get a DS bonus of +6 (10% of Pikemans DS). This, I believe, would, at the very least, encourage a combined arms strategy in armies-though I believe that stacked combat, generally, encourages such an approach.
Anyway, sorry to introduce SOOOO many concepts at once. If any of this confuses you, then please let me know and I will be happy to explain it a bit better!
Yours,
Aussie_Lurker.