Limited participants, like treasure maps (between 1-8 players, not more)
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The problem isn't that its always going to be crap, its how they implement it that causes it to be crap.
Lets start at XI's Sky for example. A 'raid' pop system actually worked very well in the open world. A Hierarchy system of force spawn mobs, each progressively harder (except genbu). The only problem with the whole system was the initial NM's to gain pops. Mother Globe, Faust, and the others proved to create competition/drama between people.
This same hierarchy system was added again in ToAU with ZNM's. The big difference was how the initial pops were purchased with a currency. Something the XIV team is EXTREMELY fond of. XIV's team could easily duplicate this system in the open world by adding additional GC ranks and allow the pops to be purchased at the max rank with some fairly high amount of seals. Lets say 250k? Nothing to crazy, but nothing absurd. After that just do the same as ZMN's. Make the next teir require a few kills from several of the base mobs, and make the drop NOT 100%. Not 1% either. Maybe 50%?
Boom. Open world content.
And lets face it. A large amount of XIV players use to be XI players. Nostalgia FTW.
Don't forget CoP; Sea had a similar system.
Still wish they'd take this system and rework it so the initial pop items are found in maps or some other non-competitive instance, like large-scale leves.
Edit: Just saw your mention of making them buyable, that works too for a nice gil-sink!
Vanguard SOH had a good solution for this, even though the game itself was buggy as hell.
MASSIVE dungeons, with named mobs all over the place, so you'd often run into other groups, however the ultra rare bosses would give you a lockout after you'd killed it once within a set period, 24 hours for regulars, and 7 days for uber raid bosses. You could still see the mob, but it would show up as ghostly and unattackable, as well as non-aggro until your lockout timer expired.
It would respawn relatively quickly too, so it didn't really exclude anyone from content.
Some of our raid nights were literally just going around to different places in the world and engaging whatever raid bosses were up at the time.
Ya the Meta events in GW2 HoT has been quite fun to get a bunch of people together and complete it. It is a zerg fest but it is a organized zerg fest that requires players to work with everyone on the map. People are even creating Taxi's group to get more people into the same map instance of that Meta content.