I like the idea, but if i remember well, in 1st Strat you were not really a merc, but just a representative of the soldiers of the commander's army (tickets).
But the idea is good, if you are near the battle may be you can join really, with our own ticket, lets say you have an army of 100 soldiers, you can use this tickets instead of commander's tickets (till commander lose all his tickets, then you lose too, or commander takes your tickets).
In last Strat there were plenty of guys with 10 to 100 tickets, this tickets were useless for randomers (all villages had around 500 tickets).
May be you can also use your cRPG equip (heirloom!) or you can only take one or two of your equip with you (this could be also a new strat skill).
But, it should have a drawback, a risk, may be if commander army is beaten you can loose one of your equip, or you have to pay upkeep in some way (5 hours siege...).
Btw, I don't know how much reknown players will ask to fight as a real merc, with their own equip...