In my opinion, the next big challenge for PvE MMOs is creating interesting content for a group that varies in size. Right now, good PvE content is built for groups of a very specific size, be it 5, 10, or 25 people. As we all know from the past few years, this leads to all sorts of complications where groups have to maintain a bench. Where people have to sit out in order to guarantee that future attempts will have enough people.
I believe this is the one true strength of Eve Online. There is always room for one more in an Eve Online fleet. This makes participating much more fun. You always get to go with the group. You never have to sit on the bench.
The few attempts in the themepark MMOs to allow a variable group tend to fail for a few reasons. Very often the content just becomes a zerg, where you just throw bodies at it. If the content difficulty is fixed, the maximum group size becomes the "accepted" size. We saw this in Vanilla, where dungeons were capable of being done in a 5-man, but everyone insisted on taking 10 people.
Ideally, the size of the group would scale from n to 2n -1. So if you wanted content for about 10 people, you could go with 7 to 13 people. This allows the group to divide into two smaller groups that can still do the content.
There needs to be some difficulty scaling involved. If there is no scaling, everyone will insist on taking 13 people always.
But scaling things appropriately will be a significant challenge. I am not sure that a game with variable group sizes can offer such a tuned experience as WoW does. However, variable group size would make organized grouping much easier, and smooth out a lot of the current social issues with grouping.