Despite the photo which appears to verify the claims made in the Tweet below, I'd caution taking it with a pinch of salt for the time being as any "Exclusive" to come from a Twitter account with less than 150 followers is a little dubious. However, giving it the benefit of the doubt, there's no denying that the information revealed here is very interesting indeed. We've heard rumours about Ezra Miller showing up in Batman v Superman: Dawn of Justice before so that's no great surprise, while him having long hair was confirmed in the promo art shown at this year's Comic-Con.
What's really interesting here is the talk of a "post apocalyptic" style costume for Barry Allen? The dark and gritty look of the DC Extended Universe means it was always going to have an edgier look than the bright red version from the comics, but this sounds like a very unique direction to take the character in. What do you expect it to look like? As always sound of with your theories below.