Well, it could just be a costume for the player character. If Sun and Moon bring back player customisation - which I really hope they do - considering these will be released in the 20th Anniversary year I wouldn't put it past them to include costumes of past characters. I wouldn't jump the gun and say it's an indicator that Red is in the game...it might even just be a placeholder character - after all, the girl in that pic looks a bit like Caitlin to me, but I doubt she'll be in the game - for concept art.
Would I want Red in the game? I dunno. I think it'd be cool if you could upload your player data from Red/Blue/Yellow as a battleable character or even as your avatar, or if you could unlock a Red costume by having save data on the 3DS or something, but...eh, Red never really struck me as a particularly memorable character. He wouldn't be amongst my top choices for cameos.