Taking into account there's, let's say, 13 worlds (counting originals and not counting cutscene only ones), for the sake of arc numbering.
We have, as of now, four worlds revealed (Corona, Olympus, Toy Story and Twilight Town), barring Cable Town and taking into account that Mysterious Tower will likely be part of TT.
Let's substract San Fransokyo as we haven't got a single frame of footage.
That makes for nine new world showoffs, old or new. We also still have to get a look on Rapunzel and co, so Tangled should feature at least in a trailer fragment. But for the sake of clarity, let's not count that.
Worst case scenario that hasn't got a game delay means we have less than 18 months until game release.
Simple math: a reveal every two months.
Realistic approach: marketing does accelerate.
Extra consideration: not every announcement will be for a world.
Let's say we have a trailer every, as an average, month and a half.
So, upcoming events:
- Gamescom (August) - Practically discarded (as Square Enix is going to heavily promote FFXV Expansion Pack
- Tokyo Game Show (September) - As Oracle stated, it's not a sure candidate. However, September is a perfect month for it, maybe as an independent trailer or anything of the sorts.
- Barcelona Games World (October). As interested I could be in it (heck, I'm actually thinking of visiting it), it's nuts to even think at the possibility. Last year, we had a little something for FFXV, but still don't hope for anything that's not a little nod to the fans. Anyhow, the independent trailer approach could work as well.
- Paris Games Week (November). This one actually could work.
- Play Station Experience / Jump Festa (December). I'm pretty sure these two could have separate trailers if they weren't a mere week away.
- Don't think there's anything on January. Correct me if wrong.
- D23 Japan (February): Well, this one's obvious.
Of course, I'm not taking into account concerts or any other kind of event that's not a Disney/Video Game event, so if there's any gap on my list feel free to add your viewpoints
Anyhow, I guess it will be monthly (probably Famitsu/V-Jump/Dengeki reveal with an international info dump shortly after) when we've hit D23 Japan. Probably going big on E3 2018.