I think the NM will be up every time the conditions are meet, with a internal cooldown after defeated. The idea should be the mark will be up many times every day. So, you find him, call for ppl and zerg it, get the reward from your quest mark, and you have the rest of the week to kill the same mark till the week reset.
It that case, the only way to make it different from FATE, is that the NM have some AoE that hit for so much that kill player left and right or have kind of mechanics that need to be coordinated. In that case, ppl will be forced to make "groups" balanced depending on NM you are fighting on.
I think they could add a lock out timer, after the NM is pulled you have like 45 sec to join the frey or you are locked out. That way it become like a hunt, be there READY to kill, not just wonder and jump on a elite mark that will mostly will kill you in sec if you unprepared. that would be so much fun and in OPEN world.