They could have "battle personality" when they step into a combat, extra "spicy-ness" comes out of them. Making the battle feel more engaging and immersive.
I still have no idea what are you talking about ...
Like while Lae'zel would be yelling "for Vlaakith" while cuting her enemies head off ...
Karlach would be yelling "Zariel will not get me" while cuting her enemies head off ...
Is that the big change?
It depends on how big change you want to make. Usually means how much money you are willing to throw at it. CRPG are interesting as you've multiple character. I would take an advantage of it. Normal Single Playing RPGs can not do it. Imho, my idea is excellent for CRPG devs. In Skyrim and Witcher, you have this one dude, not much you can do with it. Or if you want to do things bad, go with AC Valhalla style.
I think Larian is quite foolish if they dont take my idea, to be honest. My idea even has a bit of "tabletop" vibe in it. Sure they are still more Actors than Kids/Adults at table.