I would not make the weapon choices broader for merc right now as long as there is so few options for variety. I guess you should not have access to all of a weapon category with a said merc (except rhino and sparks).
I also quite agree with @STARRYSOCK about identifying your foes, but I rarely go this far.
One possible idea though would be to introduce a loadout crafting system which, as a flip side, would create weaker cards in terms of augments power and association : You can select any of the weapon the merc has accessible, but each of the three augments slots are separated in three categories : Combat, Ability, Support. This way you would not have access to combo augments available on standard cards such as focus + quick eye, quick draw + drilled or even Get up + potent packs, which can be interesting choices with some mercs.
And on top of that, if it generates too much gain over standard cards, even decrease augment values (for example, get up would get set to +20 hp on revive instead of 30 hp).