I really enjoy asynchronous warfare games, so I've spent nearly 100 hours with SK and SK: Rivals. There's one detail that seems to be missing, though: It should be possible to completely destroy Mirdoth's army in the field, to the point where Mirdoth is forced to hide in his fortress waiting for your coup de gras. For some reason, the more successful you are against Mirdoth and the deadlier your threat, the larger Mirdoth's army grows. During one playthrough, I basically barricaded the exits from Mirdoth's fortress with arrow towers, yet powerful stacks kept streaming out of his lair. Shouldn't there be a limit to how many units Mirdoth can throw at you? Also, once you reach the maximum threat level, the Sorcerer King's forces should stop attacking the shards at all and commit all available units to destroying the player. The ability to destroy the enemy army in the field and to distract and weaken the Big Bad prior to the final battle are among best achievements of classic asychronous war games.
[EDIT] These ideas come from my experience playing the late Mike Singleton's Lords of Midnight and reading Lord of the Rings.
* In LOM, Doomdark sends out a pall of "ice fear" that debuffs player and allied troops. By stealing the Ice Crown, the player eliminates the ice fear. Also, Doomdark's legions can be crushed in battle prior to the final siege of Doomdark's fortress.
* In LOTR, Sauron's Eye sends out a dark fog that buffs the orcs. After Sauron's army is annihilated at Minas Tirith, the dark fog rolls back to the borders of Mordor, and the Eye nervously tracks Aragorn's counterattacking army all the way to Morannon. This effectively distracts the Eye from the more urgent threat of the Fellowship's journey to Mount Doom.