I actually think that WM should be Edge vs HHH. No Hardy vs Edge, no HHH vs Edge vs Hardy. Just straight up HHH vs Edge is a huge feud between the two, because other than a breif stint when HH got drafted, they have never gone one on one. That should definately be the main event...
That being said, theys hould have Hardy win the belt (and I think he will) at Armaggeddon. You can tell by the way he was built up. Anyways, then Edge should win the Royal Rumble, but have Hardy lose the belt to HHH at No Way Out, and that takes Hardy out of the picture, and puts HHH vs Edge at WM.
Anyways, there'a alot of different option, but to answer your question, I think armaggeddon will be HHH vs Edge vs Hardy.