Then prepare to witness aimlessly wandering AI at its finest with nothing you can do about it. But, hey, you can easily cheat some cash, equipment and relations that way, so it's totally worth it.
Freelancer? I'm not a fan.
What I like about your idea, though, is campaign-long mercenary contracts. It would be nice to fully embrace mercenary career and, for example, have gossip at tavern include 'Kingdom of XYZ is preparing for campaign against kingdom of ABC. Their marshal, lord DEF is seeking for mercenaries'. And then you can just go to said lord and get yourself enlisted - for that exact campaign against that exact kingdom. Faction-neutral roster is kinda poor and full of mercenary troops I'm not interested in. It would be nice if I could lead Nord group knowing that I won't have to fight against Nords and receive morale hits.