From a strategy perspective, I quite enjoy 1upt, hex grids and the current system of ranged and melee attacks. Its fun from a gaming perspective, and if the computer AI was a little better, it'd present interesting challenges too.
From a versimillitude perspective, the stack makes a lot more sense. However Civ 4's execution of how stacks worked made little sense, with suicide catapults damaging 20 units, and a cavalry charge against a stack of twenty archers always hitting the 1 unit of spearmen present.
I think Civ 6 needs a new approach, which combines the best of both worlds. Personally, I'd like to see:
- Hex grid, for sure.
- Stacked units on a tile, but a penalty for doing so in terms of maintenance. That is, logistics wise up to 3 units could "live off the land" but anything beyond that would cost gpt to maintain. An army garrisoned in the city could/should be able to support a larger number of units for free.
- Tied into the above, the idea of supply chains: if you cant draw a clear path from a stack back to a city, maintenance costs increase. Techs and buildings would play around with this, of course.
- A stack is treated as a single army, with stat values dependent on its component. Ranged units would add to its first strike for example, cavalry to its shock value, infantry to its resilience, siege units to its siege. Morale would be vitally important, affecting up to +/- 50% strength.
- No healing in the field, and healing would cost gpt or other resources to carry out to represent new recruiting costs.
- An option to merge units of the same type for virtual healing.
Hand in hand with this, I'd like a proper casus belli system, with strong justifications for war improving morale, economic hardship and war fatigue weakening it, and diplomatic consequences for war appropriate to strength of casus belli and current diplomatic relationships. I'd like to see the range of excuses for war develop through the eras, but this to be matched by an increase in the default level of distaste for war.