Nice, but...
It won’t happen because the 4 man party is tied in with their Co-Op Multiplayer model so there’s about 4-5 other hard coded systems that make it necessity.
Only Larian said it would be easy to mod as there is support for it.
I'm all for the six party member constellation. It makes the party more varied, speeds up combat, present greater tactical play and would, to me, simply be more fun.