Interesting topic... and it won't be very long before we know the answer.  Like others, I certainly expect it'll be a new line/#1.

But which one?  There are so many worth candidates piling up.  On the one hand, there are the few remaining "SA Holdouts" -Dr. Doom & the Watcher (which would make a GREAT done-in-one ala Black Knight/Yellow Claw), Ka-Zar, Captain Savage, NBE, and also the various significant SA Western titles like Kid Colt, Two-Gun Kid, and Ghost Rider.  On the other hand, we've been told that Iron Fist and Power Man will be coming as Masterworks, and it's unthinkable that MTU & MTIO and also (Johnny Blaze) Ghost Rider won't come as Masterworks.

And there are definitely several worthy horror titles like Tower of Shadows and Man-Thing... to name a few.

There are a lot of "worthy"/"necessary" volume ones waiting to be done.  I expect 2011 will have at least 3, and maybe even a record-breaking 4 MA volumes.  We'll see.

I agree Ka-Zar would make a smashing volume -especially if the series was done as a pair of extra-fat volumes instead of the "done-in-3" approach as mapped in the "What If" section.

Hmm.  Personally a done-in-one extra fat NBE volume would be my favorite for #150... but sadly, I think it unlikely, especially as it would require a record-breaking 400 plus pages to do as a complete/done-in-one volume -which would make the most sense- and will therefore IMO be more likely to be done as an "Omnibus" (in which case I'll sadly pass on it).

A basic alternative is either to have a "holdout" title or a "new" title... I'm guessing the most likely candidate would be either MTU or Iron Fist, symbolizing moving towards Masterworking the "future"... although I'd prefer to see a "holdout" title -like a Dr. Doom/Watcher or (Western) Ghost Rider volume, symbolizing whacking out the last of the "holdouts".

My best guess is that it'll be MTU or Iron Fist.