How about a map with no timer?
Seek inspiration in the 'Battleground' map from Dark Age of Camelot lvl 20-24. Imagine a landscape with a hill, and a capture point on top.

If you hold the point, you get the capture bonus, otherwise just roam around and kill. When your done, exit at the door to leave with your xp and gold.

Seems more like a boring point farm that promotes imbalanced teams 24/7. If you let players log in/out at will and they get to keep their points, you suddenly have massively fluctuating team numbers on both sides. So it's going to be a total landslide for one faction or the other (losing side decides to leave en masse). And the rare occasions that you get good even fights are ruined when a few players on one side die once and log off.

How about we keep the idea of no timer, but instead of the match lasting forever there is ONE objective for each side. It could be a symmetrical map of who can cap whoever's base first or something. The important part is that players still can't leave with their points like sore losers. The downside is that matches could last too long, which is bad for people with schedules.

The mode you're describing is King of the Hill, a singular point contested among two or more factions. It could be great, but there would certainly need to be a meter or something to track the winners. The game isn't really built for the entry/exit method you describe. The match would have to end eventually.

I would actually want a battle royale style map where we can even be pitted Eldar vs Eldar, LSM vs LSM every man for themselves there can only be 1.

Back in the Dark Age of Camelot days there were an issue with the Albions having a larger faction, and so most often would own the hill.
It then became about trying to lure them out and smack them, or, leave them alone and circle around and fight the Hibernians, or, attack when we saw the Hibs attacked from the other side.
It was fun!

The issues you mention come with an open world, but it does not mean its stupid.
I doubt you will have large fluctuation, im sure the games will fill up as fast as the current ones do.
There is no reason for the battle to end. It will last an eternity (crusade), or as long as there are soldiers.

I think it would a cool way to try an open world, even if you only let in 15 guys from each side, (+ some tyranids in the castle.)