MS has confirmed to be holding a conference at Gamescom. And there's apparently some reports that MS will show off a new title (many suspecting it's Crackdown 3). And if I recall, during Halo fest '11 (which was in August) we got a concept art trailer for Halo 4, we are nearing the same point of time in development to see a concept art trailer. And if not at Gamescom, I hope we see something at some point this year. I remember how long it took 343i to show off Halo 4 after it's announcement and it was a horrid period of waiting (For Reach and Halo 3 we saw first gameplay during the same year as their announcements, Reach at the VGA in the form of a trailer and Halo 3 in the form of a Alpha-stage vidoc 'Et Tu Brute?'). So we might see a concept trailer or gameplay trailer (if we're super lucky!) at gamescom. PAX is more realistic for a concept art trailer though or gameplay their if we are lucky. Almost 100% chance we will see some gameplay at e3 though.