I really doubt we'll see a Trump-Cruz ticket. It would be seen as an absolute betrayal by Cruz's supporters and would likely cause friction throughout their campaign as Trump moved to the Center on issues. Not to mention damage future attempts by Cruz to run for president.
Betting odds have Kasich or Christie as significant possibilities. Either would be good at getting independents on board but knowing Trump he could pick absolutely anyone. Nikki Haley could turn Hillary's demographic advantage against her. Though Nikki recently endorsed Cruz...though that could be strategic since if she flips to Trump it'll look like an endorsement from Cruz's slightly less nutty followers.
Come to think of it I think i'll throw some bucks on Haley getting it.