I'd say it's highly unlikely to be on Steam.
It made some sense for Beth because Skyrim is already exclusively Steam, and because of the sheer number of mods created for TES games. I got a strong impression that one of the reasons for the Steam agreement was to prevent anyone else from doing it in a more-rapacious way (i.e. trying to sneak in "charging for mods").
Also, the integration just between TW2 itself and the Steam client has been, to put it kindly, problematic since day 1. The steam overlay has been on-again-off-again with every version (off since September), and every patch update has had serious problems. There doesn't seem to be a whole lot of communication between Valve and CDPR.
And what Mister Mida said. CDPR have a very laid-back attitude towards modding (the total opposite of Beth, who like to keep an eye on it and exercise some control). Steam just doesn't seem to fit into that mindset.
Just to put things into perspective, the top Skyrim mod at The Nexus has more downloads than the total GAME sales volume for TW2. It's a different ballgame.