As Toast said, would minion spawn frequency and amount scaling with the group size be something you’d want to see? I do want to add that if we do it we’ll also likely tweak the arena shrinking to make sure there’s some urgency even if you know what you are doing.
Regarding HP Scaling, let’s do some experiments:
Let’s take the Void Entity (400,000 HP) a Warrior with top of the line gear, to him it would take in theory 88 seconds for him to take down the Void Entity. Now what if we added the lowest realistic DPS to his? Enters the T8 wearing Priest with average DEX and ATT!
The Void Entity now has 480,000 HP and the TTK when you combine both (assuming Berserk is given to the Priest) is now 98 seconds, clearly that means it’s a bad thing to have additional people, but what if that priest was maxed? Well a maxed priest with T8 gear and berserk alongside our top player warrior would result in about 88 seconds. Again this is basically the widest gap and is theory, warrior and priest might not hit all the shots, the priest range could be useful as well as the heals which would make up for the longer TTK but let’s put that aside for now.
Let’s try the exact same thing but with a T8 Wizard with average stats, the TTK is now 95 seconds, however if the Wizard’s DEX and ATT are now 55 each, the group kills the Void Entity slightly faster at 87 seconds! As you can see in general unmaxed classes might not be as punishing as it might seem on paper but let’s keep digging.
Now what if our group was 3 people? First off the Void Entity would now have 560,000 HP and to add to our top decked Warrior, we will add a top decked Paladin! Their TTK on a 2 player VE is about 40 seconds, now if we add the average stats priest to the equation and again assume he’ll receive both buff we get 44 seconds, as you can see the difference is a lot smaller this time around, but he could still be considered dead weight, now what about a maxed Priest with T8 gear? It takes about a second longer and once again this is ignoring all the priest advantages as well as the fact it has the lowest DPS of all the classes and is bound to run into those sort of issues.
Now what about our wizard? Let’s try him again! With average stats it takes only 2 seconds longer to defeat the Void Entity and with 55/55 it takes about the same time. Again we are overlooking the advantages of the classes but even in what would be the worst case scenarios the threshold to be useful isn’t actually as high as you might imagine and relies more on the actual class stats and weapon type than maxing.
TL;DR: Yes unmaxed character can negatively impact your group if you only talk about raw DPS but even in the worst case the difference isn’t as high as you think and that’s overlooking things like range and other advantages those underleveled classes might bring to the table.