Adventure mode is available now btw. I tried it yesterday; you just choose it at the world crystal and can switch back to campaign whenever you like from there too.
Looks like a handy way to level past a difficult area or boss, which adds credence to the idea that they’re using scaling as a gameplay mechanic.
Also gives you a chance of seeing new things in an old area. I’ve already come across an NPC that didn’t appear in my campaign. Pretty cool, especially given that it’s just a few weeks after launch.