A lot of people have been advising Rubio to drop out and endorse Cruz in return for running on a ticket with him. What I haven’t seen many people weigh is whether Cruz would be better off taking Kasich as his running mate. Three arguments for Kasich over Rubio: He’s more popular in his swing state than Rubio is in his; Kasich is less ideologically (and biographically) similar to Cruz than Rubio is, and therefore might do more to cover various weaknesses; Kasich has said fewer sharp things about Cruz during the primaries than Rubio, who has repeatedly called Cruz a liar. (In this case, I see that third point not in terms of Cruz’s feelings but the political awkwardness of the ticket.) One argument for Rubio over Kasich that I haven’t seen elsewhere: I think Rubio would be more willing to subordinate himself to Cruz and less likely to freelance.
I vastly prefer Rubio to Kasich myself; but that doesn’t mean he’s the obvious choice for Cruz.