Every Civ game has done a decent job of keeping wars to the strategic rather than tactical level. However the unit design, from the publisher, needs reworking as does parts of the combat system. From my perspective, war and armies in Civilization are all seen and deployed on the grand strategic level. This means that the units you see must be at least of the Divisional, if not Regimental or Corps, strength and organization. Very few divisions in history were composed of just one or two kinds of units. Missle and melee units would have been integrated at this level. My solution is inspired by Hearts of Iron and based upon its answers to these questions.

Instead of building Spearman, or Archers, or Swordsmen, you would have the option of building one of a few divisional archetypes. Along with choosing the archetype, you also can choose to attach a sub-unit to the division to make it more effective at some task. Attaching sub-units makes the unit more expensive however. The assumption behind this system is that these major divisions are mostly composed of a couple tactical types and have various supporting types integrated. Attached units usually are of the supporting rather than core type. Eventually air or even naval units could also have this same system. I will list examples of what I mean a couple posts down.

Well, AFAIK CIV4 already allows unit joining. If they also implement a combat logic for joined units similar to that of Army in Civ3, that would be probably what you wanted.

Imagine a 'unit' consisiting of 5 joined swordsmen and 3 spearmen. Should cost like hell but should be invincible for any usual single unit (of course, enemy will also surely have its own joined units though).

Although I see what you mean, this is not what I meant. An example would be a Light Infantry division would cost 30 shields(think spearman), but you could make it a Missle Divisions for 40 shields. That means it has more defensive bombardment and the assumption is that more missle units are integrated into the division and the tactics they are taught are more situated towards that kind of combat. Of course when I list what I mean, maybe it will be more clear. Also, the army to army combat system in Civ 3 sucked. I would prefer a systme where each unit in one stack picked a random unit to fight in the other each round, which some prefernce going towards whichever unit had advantages and which side had initiative.

Arguably, they are moving in this direction with the greater emphasis on combined arms in Civ4. However, rather than having a single unit to which various upgrades are added, the different types of specialties are modelled as separate units. In some cases, you could use those units separately, such as for specific purposes, but you would be foolish not to deploy your units as an army of different kinds of units that fight together.

My point is that at the divisional level you incorporate most of the 'speciality' units mentioned in civ. Few divisions are solely anti-tank guns, greandiers, or archers. The only exceptions are strategic functions that the entire division is designed around such as amphibious, airborne, strategic bombing, etc. Even in those cases, it features of a myriad of equipment designed to achieve the same goals as the 'specialty' units in the standard cases. My argument is to represent scale better by dealing with division/brigade sized units rather than the ambiguous sized/roled units we have in Civ.

Sure, it is more realistic, but explain why it makes for better gameplay than the tried-and-true Empire based combat system that has worked for decades (if you include Empire and its variants).

(Just to be clear, "It's more realistic" and its close brother "It's more historically accurate" carry nearly zero weight to me with respect to what I like in games.)