A roaming monster is a normal mob, like a marmot. SE plans to have such mobs turn into NMs like happened in XI. Leaping Lizzy type NMs.

The current NMs are a joke but the game does not need any 24 hour spawns. You will be there a very long time even if the drop rate is 100% if you're camping against half a dozen linkshells every night with half their LS wanting the drop. No thanks lol. I'll only accept 24 hour spawns if they drop second rate gear that is worse than anything that can be obtained in an instance.

Open world NMs are always going to require little skill because open world fights are limited in what can be done and anyone can go zerg, die, zerg, die until the mob dies. You can't do that in a timed instance because if you suck you won't even make it through to the mobs that drop the good loot or have the time to kill it if you do get through. You can also have more dynamic effects in your instance that just wouldn't work in open world.