Open raid-instanced with a long CD should be considered, imho. I can see both sides of the argument though. Making end-game a semi-"open" zone kind of deludes the sense of accomplishment in one's guild, but you're correct in the stagnation caused by there only being a handful of guilds that can accomplish these feats by themselves. Cooperation leads to more successful attempts, faster gearing and a generally happier player base. It also leads to faster conquering of content, and more boredom once said content is finished. It's hard to balance it both ways.
With the current population numbers, I can see it being very difficult for most guilds to accomplish what they want without each person boxing a full group. And the new information on VT does make DSK3 a lot more relevant, because the gear is kind of "meh." An open instanced style of raid would be a boon for those of us that don't box an army. "Ok, our raid has 5 spots left, adding 2 more toons won't hurt and heck, actual players are *usually* better than scripts."
If something similar to what I think of as an LDoN raid concept is used, the instances would have to be truly sadistic and frightening

Personally, I'd also like to see a strict player limit involved as well. Say, 4 groups max? And balance said content off the max amount of players allowed in zone. This cuts down on players boxing 3+ groups so as to allow other guildies in the raid. It also helps spread out guilds a little. In the end we know very little of what those evil devs are planning, other than they're working very hard to make sure we die

And death is something I find fun. If you win 75% of the content you're playing, you're not playing hard enough

(which makes Bate the hardest player of all!)