IAmBrook had some interesting points. Personally, I don't think they will make #150 an Atlas or Golden Age volume -- for one thing, the December release is typically a single, Marvel Age volume -- so I'll go over each of Brook's Marvel Age titles assessing the likelihood:of that title getting the honor in, oh, three weeks or so:

Tomb of Dracula - Given that the second volume of the Omnibus is still on sale, this is highly unlikely
Marvel Team-Up - The most probable, but it seems that they'd want to get Spider-Man and MTU on an every six months frequency, and this would be too close to the last Spider-Man release. Also, remember that Spider-Man was volume 101, so perhaps they would pick the Spidey team-up title to start towards the next milestone with volume 151.

Not Brand Echh - My preference, and something that I figure is actually fairly likely. Especially given that about 15-25% of the material has already been printed as bonus pages in Inhumans, Avengers, etc. The only factor to me that makes this less likely is that it would presumably be a done-in-one, and the two milestone volumes so far have been for long-running series (Captain Marvel and Defenders).

Doctor Doom - Highly unlikely. There's only the MSH story and 8 10 page stories from Astonishing Tales, unless you're either gonna jump ahead to the mid-seventies with Super-Villain Team Up, or re-present already-Masterworked titles.
Iron Fist - Possible, but I like Brook's point about keeping Spider-Man, Avengers, and FF as the leaders of the non-mutant pack. I give this no better than 30% odds.
BA Horror - THIS sounds quite possible, combining Tower Of Shadows & Chamber Of Darkness, and perhaps taking advantage of the EC Archives hiatus and the success of the Warren Archives. Only two negatives: possible licensing issues, and the fact that it's a short run.
Ka-Zar - Believe it or not, right now I'm thinking this is about the most possible. It's a character with ties to the X-Men, as well as to the Golden Age; it's a character that had some success in the sixties, seventies and eighties; and this volume would include 1960s work by big Marvel names.

Marvel TIO - Unlikely. Too far into the seventies, and I think they'll start MTU before they start this series.

Invaders - Highly unlikely, given the 3 softcover Classics volumes that didn't seem to sell too well.

PP - Not a chance. They'll start MTU before this title, I'm certain.

Man-Thing - Although I'd personally love this, I think it's more likely that Ka-Zar would come out first, to introduce the character. I DO think that this will be the first Marvel horror character to be Masterworked, though, when the time comes.

WW by night - Unlikely. I think they will finish the TOD Omni line first, and then introduce Man-Thing in the line. Even though WBN predates M-T, it is definitely a more minor line.

Captain Savage - Not so close to Sgt. Fury #3.

Ghost Rider - Possible, but again following Brook's point about what the leading titles will be, I think it's a little premature.

Moon Knight - WAYYYYY premature. You'll want to get WBN Masterworked first.

Luke Cage - Possible. The timing is about right (Hero For Hire #1 is cover-dated June 1972, so the three major lines have (barely) reached this point).

So, bottom line, the most likely are Ka-Zar, Not Brand Echh, Marvel Team-Up, and bronze Age Horror.