I also wouldn't mind permanent Anti-Insurgency units for that same reason. However, I don't know how much insurgencies will flourish in future conflicts. Perhaps having a small unit for each branch would be enough, who can then train more numbers when they need to.
However, I don't think peacekeeping and anti-insurgency operations warrant separate military branches. Rather, I think peacekeeping should be handled much like how we handle special operations. That is, each branch of the military would have their own separate Peacekeeping units and those units will train and work together more through interservice cooperation than they do with other units of their same military branch.
If we're talking about adding more service branches, though, I would like some research done in the virtues of having a separate Cyberwarfare branch of the military that works with the other branches in developing attacks and defenses for cyberwarfare. I really think that's going to be a future arena in warfare and will eventually warrant it's own separate and independent service.