Questions:
1. How is the player supposed to know when a town needs his/her help? Messenger? MessageBox?
2. What are the consequences if you fail in defending the town?
3. What are the rewards?
4. Are you to leave the Mercs posted in a town to keep it safe, or bring them only when there is an attack?
Those are the kind of things that need to be sorted out before i could start coding.
1.Hmm, do you know how you get rumors from the local tavern? Perhaps there could be a main quest for each town (like the thalmor invading Windhelm, some bandits attacking riverwood and falkreath, some giants and Whiterun, etc. Could be presented as a bounty note, that asks for assistance in defending the hold. after accepting the quest you get some guys, go outside, talk to a general with a few guards, and then the town gets invaded. Afterward you could talk to the general (could be Saad, or other bosses) and you could post some of the mercs you don't need to guard the town until the next raid, which a messenger could bring you a letter from Saad asking for your help again.
2.If you fail in defending the town, prices should rise for the mercs, and if there ever would be a loyalty scale, your mercs would probably have to trust you less. (also if possible, the raiders would scale to your level, so its actually a threat, but with worthy rewards.)
3. The rewards could possibly include, lower merc prices, standing would improve with Saad or merc faction, and you could use something similar to IM THE THANE I DEMAND YOU LET ME GO AT ONCE, for one time after each raid. You could request armor and arrows for your mercs, and get increasing pay for your help, (which could decrease with failures)
4.You could talk to Saad to leave the Mercs posted in that town when you dont need them, pay could decrease by half, and you would still see them doing radiant tasks, you would have to pay for the mercs again to get them on the force, so you could still feel powerful, but dont have to pay as much to have them around, also instead of just firing them, your standing could improve with them and you can still bring a small force with you when you travel the countryside.
I also apologize if I am asking the impossible, I just think of every possible combination so you can choose. I would like to help, but my gaming computer is currently in storage.