My dream would be procedually generated terrain for parts of the big landscape zones. This will make every playthrough fresh, while still keeping the major plot, hub and boss zones intact (i.e. permanent in whatever version you make them final in).
RMG for big forest zones for instance will feel amazing - like a real forest you can actually get lost in (though not at the scale of a real forest obviously). Think Schwarzwald or something. You Germans are the masters of lush, mysterious (RPG) forests, in my opinion, so this can only be epic.(you got this heritage at least since Caspar David Friedrich, and due to your amazing nature). Keeping important parts of zones intact will make the entire map design a jigsaw with some permanent pieces to keep it all together and some randomized ones to spice it up.
I'd say also increase zone size by around 20% for outdoor levelling zones. We need to erase the pain of waiting for respawns for a simple quest or even forcing people to play together. Think of how if you miss one quest mob atm, you may have to slow your allies down by waiting for respawns - very unimmersive and not very fluid. To spice things up, add more randomized events like normal mobs patrolling (wolf pack/bandit group/razormane/magnathaur/etc).
Finally, keep the limit of 5-6 people in the dungeons, but - maybe with a party system - allow player limit to increase to 12 or more for a living multiplayer RPG environment.