Making them harder is a better idea than making them easier. If people can't do it at their current skill level, they'll have to get better. I think the way to go with world bosses is to make it very easy for you to get yourself killed, while making it hard for you to kill others. So any mistakes you make will result in your death, but won't necessarily mean your screw up gets anyone else killed. This also keeps jokers from deliberately screwing up your attempt.
Of course, you'll need enough people that can do the job properly, and that might make it difficult for some time, but every faction will get there eventually. I've done a Sha raid where there was only 28 and we pulled it off by having enough people who weren't just blindly mucking around.
One world boss like Sha of Anger to function as a VoA replacement along with another that is still up (nearly?) as often while requiring a much more organized group is probably one of the better solutions for the world bosses.
I don't think that rarity due to a long spawn timer should be a notable factor anymore in this time and age. It works in other RPGs but has many arguable problems when it comes to WoW. One of them is better blocked off by being challenging, without hopefully encouraging the stacking of 200 people.
with tag to faction, it makes the most sense to coordinate multiple raid groups to zerg it down: it gives you 1) a faster kill, and 2) numbers against the other faction. I doubt a group of 40 people will try to wipe a group of 200.
As far as I know, Alliance has never killed him on Kil'Jaden US.We're simply far less organized and at enough of a numbers disadvantage, and most are too discouraged to try at this point after the first few weeks of heavy Requiem camp.
Anyway, I think the world bosses are already a clusterfuck, but that doesn't mean they aren't easily killable, since Blizzard didn't implement anything to prevent runbacks.
Seems Oondasta will be a C'thun. I'd imagine on a pvp realm, enemies will jump in to add to the Spiritfire jumps.