Let mercenaries level up with you.

It's a bit sad that mercenaries can't level up. They all have pretty unique personalities and it's weird to have to think of them as pieces of gear you replace rather than characters you recruit.

I'd suggest letting them level up with you, so if you find one during the campaign or early maps that you like, they can develop alongside with you.

I'd suggest that from their base, a mercenary can level up to 83 (the t16 max). Not just stat wise, but skills as well, developing into a merc you could recruit in t16 maps.

A non-infamous merc cannot develop into an infamous one, and level 84 mercs are still limited to t17 maps (or level 84 is gated behind an X amount of xp gained by the merc without dying).

Skills would probably have to remain random as they level up, as otherwise you'd get a meta of wanting to recruit a merc at the lowest level possible, which would likely be more annoying than anything else - Unless there could be an option to respec higher level mercs, but that would probably also be a bit too meta defining. Still, worth considering though. It would make mercs even more personal.