I'm voting for "other" and saying that it'll be U.S.S. Voyager NCC–74656. Why Voyager and not one of the seemingly more likely options? Because looking back at the most recent AskDST's, Voyager seems more likely to fit what DST has described than either the Reliant or the Defiant. Chuck put together a wishlist of DST's "top seven" ships stating that the list was "in no particular order," and that the Federation starships on the aforementioned list were the Defiant, Voyager, Reliant, and the Enterprise–C.
As of July 2015, Chuck noted that the Romulan BoP was originally supposed to be ready after the next ship. Supposedly, the ship was still so far from being finished as of last July that the Romulan BoP just worked out as being ready before the next ship. This would imply something with a relatively complex design, especially since Zach just noted that the design still isn't 100% final, suggesting a somewhat complex vessel. Based on the implications that the next ship is relatively complex, the Defiant should be easy to rule out on the grounds that it's size and general shape should require relatively small tooling, and only four dies (two pairs, one for the top and one for the bottom,) should be necessary for the majority of the ship, which is less than any other Federation design that would require separate dies for nacelles or other parts. Likewise, the Reliant should be relatively easy to tool as well given that the refit constitution–class saucer and nacelles could easily be used as the basis for the Miranda–class design, and that the most complex part of the tooling would be the rollbar. The complexity of the Reliant looks like it should be comparable to the Romulan BoP, and that doesn't scream of an incredibly intricate design.
With what I just mentioned about the Reliant and Defiant above, that leaves the Enterprise–C and Voyager as the only remaining candidates for the next Federation starship. On one hand the Enterprise–C seems like a possibiity as it's the only one missing from the original–E line–up, and while the sound chip could be challenging, the general sculpt shouldn't be anymore complex than the Enterprise–B, and should theoretically be simpler as there aren't any major sculpting variants of the Ambassador–class that need to be taken into account. On the other hand, Voyager basically has Klingon BoP levels of complexity because of it's pivoting warp nacelles. Something like pivoting warp nacelles (and landing gear) could easily have made Voyager as tough a nut to crack as the Klingon BoP and the Galaxy–Class Enterprise–D. Furthermore, a complex design like the Intrepid–Class would explain why DST's other ship on the drawing board at the same time was the TOS Romulan BoP, as well as why the aforementioned BoP was able to lap the next Federation starship. While Voyager shouldn't have any shortage of usable sound clips, and should be easily capable of being produced with multiple variants, (Year of Hell battle damaged, "warship Voyager" with Borg weaponry, and several others,) the design itself would be complex enough to explain the delays, and would fit within Zach's comments about variants being planned. (Technically, each of the proposed ships has at least one potential variant, including the Enterprise–C in battle damaged and non–battle damaged forms, but Voyager fits the bill for added complexity and variants best.)
Based on the information that's been provided most recently, it seems like Voyager or the Enterprise–C are the most likely candidates, as the Defiant and Reliant would be more likely to be paired with a more complex alien ship, whether it's the Borg Cube, (challenging because to paraphrase Chuck, "it needs to be more than a hollow plastic box,") or the TNG Romulan ship, (Zach has mentioned that this is a tooling challenge,) or even the Klingon TOS ship, (no word on whether it'd be a D–7 or K'tinga from the TOS movies.) Given that the next Federation ship was meant to be paired with the Romulan BoP, and that the BoP was pushed ahead in the release schedule because it was ready sooner, I have to believe that Voyager is the most likely candidate with the Enterprise–C being the second most likely. The Defiant seems the least likely to me, and if the next ship is the Reliant, I have to wonder what part of the design was particularly complex, at least to the point that the Romulan BoP jumped ahead of it in the release schedule. Again, the only thing I could think of with the Reliant would be the rollbar, and that doesn't seem excessively complex unless there's something about it that I'm missing.
Of course, I could be complete wrong and the Reliant or Defiant could wind up being the next ship we get, but having reviewed AskDST and comments made around here, I have to go with Voyager this time with the Enterprise–C following it. (I don't think the demand for the Enterprise–C was there when DST began work on their next ship, but I could be completely wrong. From what I remember, the attitude toward the Enterprise–C was "wait until we have the Enterprise–B and its variants" until those were actually released, and then the demand for the Enterprise–C hit since it's the only Enterprise DST hasn't done that they have the license too. On the other hand, demand for Voyager was up there with demand for the Defiant and Reliant, and it seems like the kind of thing that the late and great Thomas Sasser would have been eager to tackle given its aforementioned complexity.)
