Kesha and HAIM really should do something together, even if it's just an album track. That'd be great.
Greg Kurstin is a must. Max Martin (and in extension, Shellback or Klas Ählund) is highly favourable. I'd love for Calvin and Zedd, and heck, Avicii and have a track. I've been getting more into him lately tbh. Ryan Tedder would be great too, but he is kinda hit or miss. Something more like Ella Henderson's "Ghost" or OR's "Love Runs Out" than Demi's "Neon Lights" for example. I'd like Jeff Bhasker too, and Matt Squire would be cool, even though I don't see it really fitting with the rest of the lineup.