I've read many proposals for complex combat systems, and how the game is going to have massive amounts of moddability and unit customization.  These sound great, but there is a serious concern with complex mechanics.

If the player is customizing his units, he needs to be able to get feedback on his designs, so he can make strategic choices on what to do next.

One game that failed to do this was Master of Orion III.  There were all these immensely complex combat mechanics, and massive customization, and you designed your ships in detail.  However, when you actually fought with them, you had no idea what was actually going on.  If your beam ships were ineffective, what was happening?  Were they missing?  Were they not hitting hard enough to penetrate shields?  Is this because your ships are slow or theirs are fast?  Should I research better weapons?  Computers?  Armour them up and go for a slugfest?  What?

So, for example, if you've got two calvary units engaging, and your opponents cavalry, though weaker, wins the day because they get to hit your guys first with their spears before your units can engage with their swords, it needs to be clearly visible what is happening.  Or, say, you hit first, but they absorb your mighty flaming longsword blow with their shields of negation, rendering your weapons non-magical, you need to know how that is happening.

This applies to many other features as well, but I think combat deserves special focus, though.  One thing that could be done is you could recieve an after-action report, where you get to see more details on your opponents' troops.  If you have, say a scout hero, he could recieve significant bonuses.

Of course, I don't know how the developers plan on tackling this problem, but I hope to find out soon.