What if we could utilize the unassigned military units during peaceful days? A soldier can have a civilian profession, too.
Like a couple of military units could be assigned to support any work-yard which would decrease the production time until the next battle. This would encourage people to build more residences, have wilder and deeper control on production which would enable different strategies on production.
I would suggest to enable this function on level 3 buildings and up to encourage upgrades.
As the primary occupation is to be professional soldier they would not be as efficient as a working settler however 2-4 units could easily produce the same amount (either increase the quantity or decrease the production time)