Nick Jonas isn't very big in the industry, and Demi Lovato is taking a massive break, so they're very unlikely to happen. Let's not forget that Ariana DID collaborate with Justin Bieber on What Do You Mean.
I think Ariana collaborating with someone really big like Justin Bieber would be really good for her career, but I don't know how good the end result will be, as Ariana is a much better vocalist than Justin.
If I had to list any artist to collaborate with it would probably be Lady Gaga, but I don't honestly know how good that would turn out - their voices are very different.
Yours Truly (even though Dangerous Woman is a very close second). It's the most cohesive of the three, and I believe it has the best music - Honeymoon Avenue is one of the best crafted R&B songs I've heard in years. And don't get me started on the beauty of Tattooed Heart. So yeah.