Japanese preview information for recent Persona games has been coming in waves, meaning that the closest game to release is the one that gets the lion share of online and magazine coverage. So far, this has been true with both Persona Q and Persona 4 Arena Ultimax; January-ish to June was mostly Persona Q time leading up to that game's June release with a bunch of mini-trailers and preview articles and now since May-ish onward, the console port of Persona 4 Arena Ultimax has been getting that same level of attention. That game comes out late August in Japan, so if Atlus continues that pattern, I would expect that Dancing All Night would be the next game to start getting heavily marketed, possibly starting a few weeks before Ultimax's release.
What happens from there, I'm less sure of. Atlus has traditionally not had much of a TGS presence, most likely because a lot of their games don't demo particularly well in that sort of environment. Catherine was the one major exception in recent years from what I can recall, but most everything else that had a long enough period between announcement and release to overlap with a TGS, I believe, hasn't been there. Persona 5 could be an exception, but I also think that the brand cachet is large enough over there that they can get away with it being a no-show and, if they really are still intent on a winter 2014 release in Japan, unveiling stuff on their own time. Knowing that history, I would honestly peg Dancing All Night to be the more likely TGS appearance if I had to choose only one. And I think for the sheer sake of keeping public attention focused squarely on one game at a time, Atlus is probably not going to promote Persona 5 simultaneously with Dancing All Night, or at least not until the end of the latter game's campaign. I feel like it'd just be really easy for them to deflate the anticipation for Dancing All Night with people who might only have a casual interest in the series if they're being immediately reminded that a sequel is coming up as well.
Hard to say, but that's my bet as the guy who's been subtitling and translating a lot of the preview material related to the series this year thus far. Honestly, I'd also love it if Atlus just kind of released it without much warning and made people figure out for themselves what it was. I know I'm going to buy it one way or another and I miss those days of picking up a game and just having to trust based on the name it's good. But that's just me~