I'm taking the "Elements of Tactics" article by Brian Gleichman (on the links page) as a good guide, and used it to reexamine our combat system. Here's some thoughts:

Element #1: Resource Management

Traditional real-time strategy games have a few major resources.
Wheat, Wood, Vespene Gas, etc. We might try for something similar.

Element 2: Dissimilar Assets

Just like D&D, we have casters and thieves and fighters, but these niches aren't as solid because the PCs can mix and match their skills. To compensate, maybe we can make weapons that are dissimilar from each other in more ways than just damage. Weapon speed, required "swing-room", reach, and damage-type (slash, pierce, blunt) are some cases I can think of. For example, a dagger may do only 1 damage. So why use it versus a longsword? Faster speed, much smaller room required (because it's a stab, not a swing). Thieves could dart in, stab, dart out again .. help the fighter maintain combat advantage while not getting into a hideous amount of danger. The heavier and better weapons would require more fighterly training.

This would be true, too, of the spells. They should be very dissimilar from each other. There should be a FEW "weapon-like" spells, but not too many (since, if that were the case, the
character should just take a bloody level of fighter). Instead, the spells should work in different ways.

Element 3: Maneuvering

- The combat system (v39) I'd posted earlier takes a giant step in this direction. Graduated health is meaningless if the weapons affect the bars in a static, unchanging way (which is why I came up with the system in the first place). In v39, the combatants MUST maneuver in order to stay on top of combat.

- We can make maneuvering even more important by taking dissimilar assets into account. In v39 Combat, normally you simply hit an enemy to steal a point of advantage from him. However, we could rule that you don't gain the point if you don't have full swing-room for the weapon. (i.e. he loses combat advantage but you don't gain any). This would mean that two equally-skilled fighters wielding longswords against each other in a narrow hallway would mostly do glancing blows.

UPDATE (Dec 20)

Some games have armor that absorbs damage, while others have armor that makes a character harder to hit. For the sake of "Dissimiliar Assets", we could have BOTH types. Leather armor, etc. would absorb some blunt, and make the character harder to hit vs slashing. Full-plate would absorb some slash, pierce, blunt, and make the character also harder to hit. Armor should have a limited lifetime too, making it another resource that the characters must manage.