I feel there's a difference between something coming off as an expansion pack and something coming off as a full sequel, even when we're talking direct sequels.
I'll use two Western examples since it's a bit rarer for this kind of thing to be done in Japan at the same frequency.
So first up, Arkham Asylum to Arkham City. This I feel is a good example of something that feels like a full sequel. The game starred the same main character, many of the same villains, and also had a similar combat system. However, you were presented with a map that was entirely new and five times larger than the original. The combat system had a whole bunch of new options added along with giving you a bunch more options at the start. There were lots of new villains and a new character you could play as with a new playstyle. The game's structure was much more open and added in a lot of sidequests that generally had solid effort. While it had the same art direction, the visuals had a fair upgrade over the original. Returning characters often had updated character designs. There were a bunch of new enemy types to contend with and the boss battles had new paradigms. While the game would be instantly familiar to anyone who had played the first, everything about it would still feel like it was really trying to take the series to the next level. Now, some people didn't like these changes, but the game was incredibly well received both critically and financially.
On the other side, let's take Saints Row 3 to Saints Row 4. Now, from Saints Row 2 to Saints Row 3, there was a pretty substantial upgrade that was not too dissimilar from what I just talked about with Batman. However, for 3 to 4, the game used the exact same city, looked incredibly similar visually, and basically layered superpowers and some new enemies on to Saints Row 3 (along with a few other, smaller things) and called it a day. Obviously there was a new plot, but for the most part, this could almost be seen as a large expansion pack for the previous game. The game reviewed okay and sold all right, but it was down from Saints Row 3 in both respects, because people who looked at it (or reviewed it) could tell that it was a low budget sequel meant to give people who were okay with more of the same something to do since they didn't have the resources to make a full fledged sequel due to THQ's impending bankruptcy.
To me, Bravely Second comes off a lot more like Saints Row 3 -> than Arkham Asylum -> Arkham City. As someone who liked the game a lot, it still interests me, but I'm not going in with any impression that we're talking about a notably impressive upgrade to the series.
You even mention that it seemed to have a long development cycle of 2.5 years. However, here's a quote from an interview with Silicon Studios:
Source: http://web.archive.org/web/20140326...what-lies-ahead-for-the-bravely-default-team/
(I have to use the wayback machine since CVG got shut down and all their links are broken/pointing to the wrong articles.)
The game will only be in development for around 18-20 months total since they weren't even interested in starting the sequel right away after its initial performance. I think that says a lot about Square Enix's excitement level.
