Well, you can take a look at the
debut trailer for DR2 to get an idea.
That wasn't bad at all, cool. I have to imagine DR3's debut trailer will have a very different format, though, since DR2's seemed to basically be a gameplay overview/walkthrough. I would expect DR3's to look like a modern Ace Attorney game's trailer.
Edit: Though looking at other DR2 promotional material, it seems the danger comes from when one's being too analytical about what they contain. Then again, my current knowledge might be coloring it, so maybe the quick scene flashes won't be a problem going into a new game.
Edit 2: But, oh man, now that I think about it, just knowing new mechanics could be a bit of a spoiler. It was a really cool surprise when I was playing DR2 without any knowledge of it and a rebuttal happened during the first trial. Was not expecting that
at all and it was a really cool moment.