Class: Classy Race: Not Black
You have to create another character and choose to be a worgen or change your race to worgen if your class is compatible with that race.
Almost all classes can participate in the Battle for Gilneas in World of Warcraft. It does not matter what class one's character is; As long as one has a character that is in the Worgen or Forsaken race, one can participate in the Battle of Gilneas.
Undead.
Class changes are not possible in World of Warcraft at this time. If you want to change your race, you can only choose from races which have your current class available. For instance, if you are a Night Elf Druid, you can only change into a Worgen, Tauren or Troll - these are races that have the Druid class available.
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Yes, the Night Elf race is an Alliance race. You do not need Burning Crusade to be a Night Elf. It is included with the "vanilla" version of World of Warcraft.
dependidng o what you like
the troll are the tallest race but they slouch so they don't seem to be the tallest but they are
No, there are no vampires, either as a Playable race/cast or as an NPC.
Currently the Undead (or the Forsaken) can be mages, warlocks, warriors, priests, rogues, and Death Knights. In the next expansion, Blizzard announced new class/race combinations and the Undead will gain the hunter class as well.
Currently, it is not possible to do class changes in World of Warcaft. You can change your characters race, gender, server and even faction, but you can not change its class. Once a Warrior, always a Warrior, unless you delete it and start over (or level another class).
There really is no answer to this as you are the only person that knows what race and class you want to be.