In the revealed concept art, shown in the link below, it is believed that this is Tyrael confronting the rogue archangel Imperius. Now, I've been digging through the cinematic trailer for the clearest frame for any hidden info, and I came across something rather interesting. Take a look for yourself at an outline I did compared to the base image. Also, note the blue aura at the edge of the flames. (No, it isn't blue fire. It holds the same appearance as the blue aura shown in Destruction's End.)

To me, it appears as though there is an angelic figure in that fireball, and I can't help but feel that it's holding a bit of a staggered pose. Does this mean that Imperius crushed one of the only ones preventing him from purging Sanctuary? Or perhaps... is that Imperius himself?

With that it appears more like he's arched into descent, which would mark willing movement and mean it's Imperius, or perhaps Tyrael coming to give the warning.
http://www.blizzard.com/diablo3/_ima...ss35-hires.jpg