There was someone on reddit who "leaked" what it was about named WhyRenektonWhy. They previously "leaked" URF/Gnar a month before they came out, and the user is deleted now:
http://www.reddit.com/r/leagueoflege...file_page_are/
Dunno if this requires a spoiler tag/people don't want to be spoiled... but just in case..
As far as I know, Kalista, an ad carry, is coming to League seeking for revenge on several champions who are marked with shrouded profile pages. Kalista has a completely new mechanic named soulbond which links you with a chosen teammate, often your support. He/she uses spears. Thank me later.
The other links where he posted info before they were announced:
URF:
http://www.reddit.com/r/leagueoflege...ng_called_urf/
Gnar:
http://www.reddit.com/r/leagueoflege..._laner/cixp00m