The awkward truth of Open World vs. Instanced, though, is that the one with better gear inevitably invalidates the other. If consistent, open-world content provided the best gear in the game, everyone would flock to it, and vice versa for instanced. Most of us are aware of instances invalidating the world, but an example of the world invalidating instances is GW2, where there is little to no reason to spam Dungeons for Exotics, and after a few patches it isn't necessary to spam Fractals for Ascended either.
The closest solution I can think of is for Open World content to follow a completely separate progression route from Instanced content. That could mean anything from being the only way to access certain recipes, farmable loot...hell, maybe even world-specific skills or something. Yes, the side effect would be "forcing" everyone to do everything, but quite frankly after the 500th hour I think most people would be willing to break their routine.
