I would like to make one prediction for the next expansion. I expect the 25-man raids to go the way of the 40-man raids. As Blizzard continues to make Warcraft as widely accessible to player skill levels as they do to computer hardware requirements they will push the raid size downwards. To compensate for the loss of the 25-man raids Blizzard will add more achievements to make the difficulty self-selecting (similar to *10*-man Obsidian Sanctum with three drakes up being the hardest raid pre-patch 3.1.). Achievements are wildly popular and are a natural path to ramping up the difficulty of a raid.
In addition, I expect the raids to scale to the number of people you bring, with the raids being 6- to 10-man instances. This is much more small guild friendly as a group of friends can still raid without having to pug any extra members. (Aside: pugging extra people isn't bad, but there's lots of resistance as well as some outright cultural hostility to pugged members. Sometimes it is just easier on all involved if you can keep the raid 'in guild' so to speak.) I firmly believe having scalable raids this way would push Blizzard to be more creative in their raid designs. Having limits forces you to search for more creative solutions. The 25 man raids where the developers assume the raid generally has one of each class encourages more kitchen-sink style design.