There are a few notable things/anomalies about the poster:
1. Megure, Shiratori, and Sonoko are on the poster. Megure was on the posters of M-1 (because there was a shortage of characters and they needed space fillers), M-2 (because he was one of the victims), M-4 (because the police were being targeted and were possible suspects), and M-13 (because the police were possible Irish suspects). Shiratori was on the posters of M-4 and M-13 (both times for the same reason Megure was on it). Sonoko only appeared on M-11's poster, for quite obvious reasons. Unless you're Conan, Kogoro, Ran, Shinichi, or the Detective Boys, you aren't given a spot on the poster unless you're really important. And for all three of those characters to be on the same poster means they're important. That's just my guess and until we actually get more trailers out and eventually the full movie, nothing is set in stone.
2. Once again, Haibara is wearing a Big Osaka jersey. Mitsuhiko decided to pair up with Genta and Ayumi (what a traitor). Not that significant, just wanted to point it out.
3. Conan has a red wrist band on. Maybe it's an admission wrist band that everyone is required to wear. Of course, everyone else's left wrist is conveniently blocked out. Not sure if this is significant to the plot. We're just going to have to wait and see.
4. Something red is sticking up from behind the score board. Judging from Ran's positioning, I'm guessing that red thing is her cell phone. After a quick search online, I found this picture to confirm my findings.
We're just going to have to wait and see if this affects the story plot at all or not.