I think that the length of tactical combat is about right. Especially when multiplayer gets really rumbling, people won't want to wait that long in-between turns while two people duke it out on some battlefield.

That said, tieing battlefield terrain to the terrain of the world map would be neat, and would allow some tactical usage of terrain. I'd much rather meet an enemy with my army of archers on a river tile, then, where they would have to reaverse it to get to my archers, than on a plain where they can charge relatively quickly.

But if the alternative is ultra short battles where who gets the first hit in counts the most, is that really a preferable alternative? Will that really feel like a better multiplayer experience than one where people choose to autoresolve most battles and fight longer important battles on the tactical map?

Making counter spell just effect the next spell would just lead to players casting a low level spell to counter the counter spell, then following it up with the big bang.

Maybe you could all spells take one turn to cast, however prepartions for certain spells mean they cannot be cast until the 2nd or 3rd round of combat. And counter would be an automatic thing based on intelligence, rather than a seperate spell. A champion would have to go into 'counter' mode, a bit like defending with shields. Whilst they are in this mode they can't cast spells, but they have a chance of countering all spells cast by the opposition (as was said, based on their intelligence). This would be cancelled if they got attacked in melee or cast a spell of their own.