For collabs, I totally agree with Tyler the Creator. He was one of the first that came to my mind.
Also love the Thundercat suggestions. I didn't think of that, but it seems like a really natural fit.
If Eminem could get a little more conceptual (less personal) and clean it up a little I think his style could lend itself well to the Gorillaz sound and world. He used to be very good about dark themes with peppy/upbeat music. Might be too late for him to really make it work with his current style though.
I wish they'd bring back Benjamin Clementine. His vocal performance was great in Hallelujah Money, but the production on that track felt lacking and underwritten to me. Or maybe the follow up of We Got the Power just ruined it for me. That's probably my all time least favorite Gorillaz track. Either way, I'd love to see another Clementine/Gorillaz track.
David Byrne also feels like a pretty good fit for them, both lyrically and musically. Dave Bayley of Glass Animals. Jack White has done some good collabs and fits their general aesthetic well too.
I also wouldn't mind seeing some queens of pop stretch their creativity a little to find a way into the Gorillaz world. Katy Perry, Lana Del Ray, Gaga, Billie, Miley. I think any of those ladies and possibly some others could be interesting additions to the mix. Billie and Gaga feel like the most natural fits.