Personally (and Ill use WoW as my basis since I actually made it to end game in wow) Id like to see a system where you cant access the new end game content unless youve completed previous end game content.
I don't think that's a very good idea. You can't force players to enjoy themselves. If the players want to try old content then they'll do it without requiring it of them. If players want to go straight to new content then why would you want to force them through a pointless time waster?
I'd like to see 2 changes made to MMORPG end game
1. Curtail straight gear progression. There need to be much less straight up upgrades. The new set of gear shouldn't make the old set completely obsolete. This creates a silly situation where most players are raiding not because they enjoy it, but expressly to get gear. Yet it's a carrot on a stick. They work so hard to get gear, but in a short time none of that gear matters, because the new gear is better. It also creates the problem of getting gear to get gear. Content needs to be more open and not so linear in choice. I should have to raid dungeon one to get good enough to raid dungeon 2 to get good enough gear that I can finally raid the only dungeon I cared about, dungeon 3.
What I would like to see is more horizontal/lateral progression of gear. If you're a DPS class you shouldn't always be receiving gear that just makes you better at dealing DPS in the exact same way. It only seems like your making progress, but in actuality the DPS upgrade you get is offset by mobs being put out with more armor and more health. Relatively, you are in the exact same spot you were before. Horizontal progression takes more creativity to implement, but solves many of the problems with straight vertical progression. By horizontal progression I mean, as an example, maybe your tank paladin could earn gear that turns him into a respectable DPS. Or maybe it offers him options where he can tank through straight up damage reduction or choose gear that lets him heal himself more to offset damage. You should get interesting choices.
2. Less emphasis on end game. There should be so many fun and interesting things to do while leveling that it doesn't feel like a grind. Players shouldn't trudging through levels to get to the reward which is end game, only to find out end game is raiding and farming over and over again. I really like FFXI's level capped content. Give max level players way to earn rewards by trying out lower level content. I like having the different limitations and strengths each level cap provided. Strategies that worked at max level wouldn't work at level 30, and that helped keep things fresh. I also thing it builds a stronger community and help new players by not only devoted more time to content they can see sooner, but letting the high and low players mix, so you don't get as many clique group or at least as strong.