Im personally not that bothered by the 4 man party limit. Dnd is roughly balanced for that party size and have sat at many dnd tables with 4 man groups.
But more options to choose from for parties is always good and gives you so many more tactical options, it would be great!
Maybe Larian can offer an additional mode for 6 man groups? Dident DOS 2 have an option for more AP per turn and the like which was accompanied with a 'the game isent balanced for this' popup if you clicked it. Something simular could work just fine imo