I agree with screenhog's three points of stuff that should be kept in mind while revealing things. Personally, I like teaser trailers or images, maybe even comics, that help give us an insight into the setting and allow us to make up our own headcanons about what the world might be like, until the actual game is made available. For example, Persona 5 recently released a new trailer, which showed us that the MC for that game wasn't as innocent as he seemed to be in the first teaser trailer shown awhile ago.
Of course, I don't expect you guys to go through with an animated trailer to get people invested in your games. However, you guys could do a comic (I imagine that might be a little more within budget), or add more journal entries to "Captain Javir's Guide to Survivorness-itivity".
I can't find where you posted more information about the actual humans involved [either it was on here or it's somewhere on the dfq2 website], but posting more about them helped me get a sense of them, their personality, what their world is like. I could almost imagine the siblings' cookie(?) incident, and I'd love to see that in-game (or for someone to draw that once the game's out).
Maybe showing any sort of symbolic thing or an image of something that happens around the start of the crew's "greater" journal [ie an image of Group going down into a dark cavern, and just beyond their sight they'll discover Important Life Changer that sets them off on the story's quest], though I guess that'd not do too much for others...
Or you could just show progress on the art, if there's any new images that you can reveal that aren't up on the dq2 website.
TL;DR: personally, I will be happy with anything thrown out, so long as it's not 'here's a sample of the battle', because I'm more invested in the story of a game than the mechanics of battle. [unless it's an action game, then I'll want to see what actual battle footage will look like]. Also, we won't know that something's a spoiler until after we start playing the game, or unless Word of God says it is. But I definitely wouldn't be happy with all of chapter 1 revealed. : ( I'm not really a fan of repetition or replaying things until i've forgotten it, so having to see chapter one again once playing will make me wish I could just skip to chapter two.
Okay that wasn't a very good tl;dr. I've spent days trying to let this question roll around in my head in the hopes that I'd come up with something good. ;>>