Seeing as the suggestion is to increase DPS slots, the changes needed would be actually pretty easy. And I mean macro-easy. Just increase the mobs HP by X amount dependant on how many new DPS are in the run. The off-dungeon where there is a significant difference (like Gubal Library with its last boss and his summoning seals) can either be ignored (so what that some dungeon would become easy, seeing as every older dungeon ends up 'easy mode' with higher gear anyway) or get a little more effort put into them.

A slight increase in damage from mobs will make up for the sudden increase in burst AoE that could wipe a crowd before a healer would even really be necessary, without preventing less-skilled groups from being wiped repeatedly. Remember, twice the DPS, twice the people with a wide range of skill in DPS'ing, meaning that the comparative "bottom-line" and "top-tier" groups are even further apart.