Would be great to fight the primals as roaming NM's, but they should leave it as open world fight, for example with ifrit, as soon as you engage him a big circled area would form with nails that comes from the ground and fire, so other players cant join but may see how you take him down, outsiders nearby the party that claimed it, are ousted from the area, idk with some kind of animation like them running away or making an emote the party members to him that everything is under control and the outsider walking away, with this only a player or alliance can take down the primal without any outsider disturbing them.

The best of it is that they can leave the same fight stage setup that we have now or make it even harder, to solve elements like trees they should put the spawn area free of objects.

That i think is far better than instance, in Sky you were locked away on an island but everyone else outside the party could see the fight from the other side, and it was great trying to see what were they using, real time strategies and of course the recognition of the comunity.