Let me tell you why it can only be Mangini:
1) The majority of evidence we have had so far has indicated him as the front runner.
2) Has been silent on new projects after he quit as a Berklee Drum Teacher which initself was weird.
3) His website after being the same format for years suddenly stopped being functional with a message that it will be updated fully in May (announcement?).
4) Preister, Lang, Donati, Wildoer et al are amazing drummers but they do not have the personalities to match the DT vibe or feel. Mangini is a larger than life character and a true rock musician, he has the look, on stage persona, is a bit up himself sometimes for show but has a certain grace and coolness where as the other guys sometimes look like a group of accountants especially during long technical sections in songs.
5) Mangini's work with annialator was limited to their song structures thus limited what he could play. His playing on James's EOP was fantastic to the point that i read when it came out everyone though the songs were off the new octavarium album thus they couldnt really pick the drumming difference between MM and MP which says a lot. His playing with Vai not only on albums but live is amazing and he is def a showman too.
6) DT cant afford a member to just sit behind a kit and play but play very well. Their shows will become a boring prog fest and the only drummer who auditioned that has the character to keep the show going IMO is Mike Mangini.
7) They only need to strike out everything that says Portnoy and replace it with Mangini thus cutting a lot of costs down! (yes, i am an accountant and im joking).
8) All recent leaks have indicated that MM will be the next drummer.
Sorry for the ramblings but the band is a business and they would have looked at this from a business perspective and most of their buisiness is live shows.