If I'm not mistaken, Rocket League is actually an outlier to the E3 third party announcement from Nintendo, as it was rumored before then, but not confirmed.
That said, I think the Eurogamer rumor indicated that Diablo III may not be announced until after E3, so it can be plausible that it may just be a no show for the time being, until they have a proper place to announce it.
As far as a GTAV port, color me in the camp that believes that ship has passed, and that I don't see Rockstar able to port it properly for Switch.
They could play a Champions Ballad, and release it on the day of the E3 direct (as it is rumored originally for a June release).