The boot in the image seems like an outdated version on Rains boot. But he has the bowguys bow behind him. And Bowguys boot looks nothing like the one in the image, but he could be wearing a story alt in the trailer.
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http://i.imgur.com/v4cAsID.png (boot image next to rains boot).
I think he's in from a fighting game story perspective. The second part is probably about Kotal Kahn vs Mileena. Kotal Kahn has a shitload of henchmen in the game (Ermac, Reptile, Devorah, Ferra&Torr, Probably Erron (he's wearing eyeliner in 1 cutscene wich is a sniper tactic I think-> possible variation)). And Mileena has Goro...
She kinda needs more henchmen for Kotal to fight in his chapter if she's going to be his chapter boss. Seeing how they've waited this long to show Mileena, I can see her getting Rain and Reiko as sub-bosses.
And when KK and Mileena face eachother you can spot thunder/rain (weather) on the ground. He probably has a role in that chapter.