Given that the loading screens are severely reduced per their own words that would not be it, probably ingame advertising? Like that agressive product placement segments in House of Cards.I don't think gaming companies would be OK with that. Also where and when would you show ads? Would they interrupt your gaming? In loading screens? Overlays?
How would that even work? Pop ups every 15 mins of playthrough? Or every time they go into the menu? Putting it over the game UI? That's one sure way of putting both users and developers off the platform.
That would be pretty unfortunate for the developers in the long run. The Spotify model would probably not cover costs in full.
I would expect everything we see in youtube.