One thing I haven't seen before in a game is rules for allowing hit and run tactics. If your faction didn't have the means for defending cities very well (no walls, for example), you would want to set up distant warning towers and units, and then try to wear down invading armies before they reached your city. Throwing fast-but-weak units at them is one possibility. Placing slower, tougher units in their path is another (unless they're fast enough to just run around your units).

Instead, what if you had units with hit&run (skirmish?) ability? This might allow tactical combat for 3 turns or whatever works in playtesting. Horsebowmen could get off three volleys and retreat (without the regular retreat losses rules). An obvious counter is ranged units. Cavalry with the charge ability would get their devastating charge, and then retreat. Pikemen might discourage that. Both sides, of course, would have the chance for wizard spells or hero spells, or if that is too powerful, you can simply say that: "It happens too quickly for your wizard to react".

The skirmish ability would require the whole army to have movement faster than the defending army. If WoM's movement rules have a wider range than MoM's, there could be several possibilities:

Hit and run, with the defender not catching up the next round.
The defender does catch up next round, but only triggers another skirmish.
Full tactical combat the next round.

I think this fits as something that could have been in MoM, and would be a nice option for differentiating units available to factions.