Agree, this must be trolling... Grant it, I think that at some point, varying the number of party members would be awesome, but there would need to be extra balancing involved. Such as increasing the number of monsters, or buffing them if you have over 4 party members. Then debuffing or decreasing the number of enemies if less than 4. DoS2 had 2 mods for this to balance the difficulty.
And just for the record, the 4 person party concept started with Final Fantasy, way back in 1987. :P
Minimal encounter balancing is necessary. Just divide encounter exp between characters who are actually present.
-Party of 2? The exp is only split two ways, meaning that each character levels up faster
-Part of 6? The exp is split 6 ways, meaning that each character levels up much more slowly.
The first 3-4 fights would be much easier/harder with a party of 6/2, but it would very soon auto-correct itself, at least to some degree. No need for any specific "lone wolf" mode.
To prevent companions left-at-camp from being underleveled to the point of uselessness, maybe auto-level them up to stay no more than 2 levels below the PC.