I'm pretty sure that whoever it is won't have a ton of input musically, but it's still DT and it's still going to be great. I'll be happy no matter who it is.
Like none. Not on this album. Let's not forget that it is almost a certainty that in a situation like this a new band member is probationary and will never to get to write anything straight away. He will basically be handed his parts and just told to play them. He can style a little bit with what he is given, but no writing credits or royalties for the new guy until he is made truly permanent. And that can't happen until after a nice long tour, which is what we will have this summer and fall. You don't know someone's long term viability until you tour with them. It's the only relevant stress test. It is my belief that the drum stuff has already been recorded and they are moving on to guitars and keyboards now. Basically the drums are written without the drummer using drum machines, then the drummer just gets to play what is given to him.
Not that I'm an expert on these sorts of things, but that seems awfully tough to believe. Your reasoning makes some sense, but I've just seen too many albums in which a new member comes in and the sound is completely changed. To use a drumming example, think of when Scott Travis joined Judas Priest and they released Painkiller--they were back in thrashy stuff and it was a big change from the more hair metal-type stuff Priest did in the 80s. I doubt Travis did that all himself, but you'd think that he had at least something to do with it, since the style was so different.
It was my impression that MP/A7X-style gigs are the exception, not the rule. After all, if the band was really impressed with their new member's writing ability, why wouldn't they be confident in writing with him, regardless of what they knew on the tour?
Plus, isn't Derek credited with a lot of the ACoS revisions and FII stuff? Obviously, he toured prior to that, but even so, MP and JP had some trepidation about him for pretty much the entire time he was in the band (which is why he wasn't made a full member for six months and they ultimately kept trying to get JR). And of course, JR went right to work on SFAM, which had similar secrecy as this album.
Not saying that the drummer necessarily IS taking an active role, but I find it foolish to rule any of these scenarios out.
Plus, if the band is apparently so pumped, I doubt they are really "iffy" about their choice. Think about it--they're pretty much the leading prog metal band in the world right now, so I'd bet a good amount of prog metal drummers aspire to be in DT. It's not like when JLB, Derek, or Jordan joined, when the band wasn't as popular--after all, there was supposedly an audition with seven incredible drummers. Sure, maybe that's somewhat hype, but I think we can all agree that the names who've been thrown out for the past 126 pages are pretty insane talents, whether or not they are the single IDEAL choice. Would we have been discussing Lang, Minnemann, or Mangini if MP had left after FII (and the band had stayed together anyway)? Possibly, but DT may have had to go with a lesser-named or lesser-talented guy, since they weren't at the top of the prog metal heap and had just released what most considered a disappointing album. So if it's someone with such incredible talent (which it almost HAS to be) and the band hand-picked him among seven of the best drummers in the genre, I'd imagine they'd feel pretty secure in at least listening to what the dude had to say with songwriting.