Would it be too hard to make retainers companions?
Give them Duty tier AI, and perhaps 1 companion at a time. Just like Chocobo, you can't see them in major towns outside of their usual retainer bell, but are visible outside of them. Would also be neat if they could ride their own chocobo in the case you choose to use a Mount, but that's just a bonus I thought would be really neat.
As for their levels, I was thinking instead of matching your own (all those wasted Ventures ;_; ) they would remain at the level you last left them via bell. Also, death. If they get KO-ed you would have to resurrect them via item the same way one does their Companions or, for extra penalty, having to trek all the way back to a bell, getting a line of dialog as per their personality, and then carrying on.
One could not have them along if they are on a Venture, and if they're selling something on the MB, it would stop. This could give players an incentive to hire additional retainers as a bonus for Square.