Arsonist
These guys have no business being mercenaries, they are an outlaw through and through. They should be replaced by their Rocketeer variant by default.
Pandour
Counter-skirms in general are a complete failure. They are way too niche and left on their own waste most of their potential fighting units that aren’t light infantry. They also have no historical or practical basis to give any intuition to their use. On top of that, they look visually identical to other skirmishers so it’s very difficult to spot them. So overall they just add complexity to the game with no benefit.
The most sensible way to rework Pandours would be to make them more of a generalist like Hajduks. They’d start out slightly better against light infantry, but become better against heavy infantry after CIR is researched.
Barbary Corsair
I’d like to see, Rodeleros, Barbary Corsairs, and swordsmen in general get a slight bonus against heavy infantry like they were temporarily planning in the last patch. It would be very fitting for Rodeleros, and Barbary Corsairs are just the mercenary version, so they could get the ability by extension. A distinct swordsman line was actually considered in the original development but ultimately scrapped. Perhaps they could revert Pirates and Wokou Pirates back to heavy infantry if they had a more distinct role like this.