Some serious sleuthing is going on in anticipation of the World of Warcraft Expansion. The EU website has been theoretically mined to find the FaQ for the new expansion, and clever sleuths uncovered the patent for the title "Wrath of the Lich King"
So far it is suspected that the level cap will be raised to 80, players will get to invade Northrend, as reported here, Deathknight will be a hero class( which will more then likely cater to hard core raiders), a new profession, and of course new rep grinds, new mount money grinds, and new dungeons. On the PvP side of things, siege engines and destroyable objectives.
In my opinion, we will also see new talents, possible reworks of the current talent tree to avoid overpowerness, new mounts, new dungeons, etc etc.
The introduction of the hero class does imply some serious implications. First and foremost, will there be a limit? If it is anything like previous blizzard titles, anybody with enough time can make it. Sure it will be prestigious in the beginning, but after that, it will be common place. Of course if it is too hard, people will complain, too easy people will complain.
Balancing the talent trees will be difficult, as blizzard found out last patch. They promised the 41 point talents would be so strong, nobody would go for a 31/30 build. They were wrong. several classes have lackluster 41 point talents.
This will also bring about lots of PvP balance issues. It is only hoped that serious players get Beta invite and can iron out a lot of the balance issues before live.