I must restrain myself to the feasible. It is HARD
It would be awesome if combat spells could react in exciting ways.
Setting trees and grass on fire. Steam dmg in a greater area when your firesword hits and icesword or when you fireball water. An earth sword grounding lightning dmg. Lightning weapons and spells doing more area dmg to units in water. Earthquake creating impassible terrain or falling rocks from a cliff. If you use life drain on a poisened unit you are poisened yourself. Wind spells moving steam and driving burning land in a direction you choose. Dust from earthquake putting out fire.
Devils in the details. Stuff like this would add a layer of magic strategy and be just plain awesome.
That would be pretty awesome! Also what RikazeMA said. In addition to magic complexity I'd really like to see same kind of thing from weapons. For example, your staff-wielding unit is attacking someone. He hits them, and his staff breaks! I'm not saying that there should necessarily be some numeral value for condition of equipment, but maybe some % chance for them to break, depending on the material. This would then simply remove the equipment from use, and decrease stats accordingly. Then (I'm not sure if this is possible, haven't played the beta long enough yet) they could continue attacking with their fists for little blunt damage!
Also special attacks depending on the attributes of the creature in question, as was discussed here previously. This doesn't have to be overtly complex and difficult, but could add a nice extra layer to combat. For example, in my favourite game "Age of Wonders: Shadow Magic", some units like hydra who has many heads, have this "spread attack" ability to strike at all opponents around them. Or and orc warlord can swing its sword around in frenzy and hit those around him. Some strong opponents can strange enemies to hurt and stun them, fliers can grasp and throw units around. Maybe some backstabbing ability for rogueish characters etc. The possibilities are endless!
However, with all this said, sometimes it's just better to keep things simple. I think that if I were a game developer, I'd pretty quickly notice that many of my ideas just aren't feasible To return to AoW: SM, the AI in that game is horrible. It can't deal with the complexities of tactical combat very well at all. I'm sure Stardock can do better, but there are limits for everyone.
P.S A little off topic, but when I still remember it, I have to ask that please, don't make the same mistake with combat that AoW: SM did. There in multiplayer all players can see the combat when anyone is fighting. Meaning that if you want to have a tactical combat, everyone will know what kind of army you have the moment your first battle begins. A horrible, horrible feature