I was having this thought today, and some times previously as well. Back in the day mercenaries were used a lot, iirc, as well as in numerous fantasy games. At the moment the mercenary ability allows for extra armory traits (roughly speaking).
Would it be possible/interesting to have some more -actual- mercenary mechanism implemented into the game? Such as when going to battle with faction A, maybe you can take a single unit worth up to 10% of the total battle value from another faction, say faction B (so 1 unit worth 50pts max if you play a 500-pts-game). But maybe as trade-off, you give away some of your progression points towards faction B (where you hired the mercs from).
The thought of running a mercenary faction that others can also use sounds pretty cool to me. I was thinking about some generic humanoid kingdom that hires large berzerking sharkmen from the biest domain, for example. It'd make (some?) sense to when you want to hire some unit outside of your own faction, that that specific unit would need to have the 'mercenary' trait.
Some food for thought? I'm curious to know what others think of this : )