I like the "base" idea you bring up, but i think it would prolly be more plausible if the change was actually to allow the dungeons to "switch" to accommodate 4 dps or 3 dps 1 tank/healer parties. As an example the "traditional" setup is designed around one player taking relatively high quantities of damage from a bosses unavoidable attacks (ie auto attack), but in a 4dps setting the dungeon could shift to a focus on telegraphed attacks with consequences being higher for failing to avoid said damage. (potentially even add a castbar of sorts to auto attacks that you can avoid). Course this system has to also be able to accommodate to 3 dps scenarios with 1 tank or healer, such as forcing the tank solo gameplay to revolve around survival and cd usage and healer solo around "spot healing" centric measures as a base.

Did not really bring up trash pulls cuz even now if you handle a pack at a time you can dps it down while being relatively safe, could also result in interesting use of CCs that atm are quite frankly useless. (and in most cases blatantly immune)