For tacticle battles you could have a Dom3 system, but on Dom3 you normally scripted your armies in general terms, you don't get to see the battlefield's terain or your opponents army, you can't tell your archers to focus on those glass cannons, or you knights to flak that infentry unit.
Instead how about the players enter the battlle field map and order their units around, these orders (along side scripts because as we all know the plan is the first casualty in any battle) are e-mailed along with orders on the strategic map and the next turn players can see what happened, if the fight hasn't ended they can update everyone's orders.
This is actually how I think battles are planned already, when the devs say simultanious turns I think that means you can only ajust your orders at the end of a fixed turn length, pausing whenever you want is pause-based real-time combat, althought PBEM will probobly need longer combat turns.