Well, Dolph has until July to cash in, so I definitely can't see Del Rio's reign reaching July. I'm definitely thinking Dolph Ziggler will be cashing in on Del Rio, but I'm not exactly sure when that will happen. I'd love to see Dolph cash in at Mania, since that's never been done before, and I could see WWE doing that though it may not be the most likely option (someone who's watching would have a better grasp on that then I would). However, because I like the idea and it doesn't sound outside the realm of possibility, I'm going with Dolph cashing in successfully on Del Rio at WrestleMania.
As for who Del Rio will face at WrestleMania, well assuming all the dirt sheets I've read are true (Orton vs. Sheamus, them souring on Cara, the aforementioned latino one), then perhaps Rey Mysterio? Actually, due to the numerous times that happened, I don't see that happening (nor would I want it too), perhaps Rey vs. Del Rio vs. Cara? Not that I want to see that but honestly I can't think of any heel on the Smackdown roster that would work. The best I can come up with is Wade Barrett, and I'm more against that idea than the latino triple threat. Thinking about it now, when are Mark Henry & Christian returning? Either of those two could fit the bill. Especially Mark Henry, going by the fact I want Dolph cashing in after the match. Definitely don't want to see Del Rio/Henry, but Henry can definitely make it an opportune moment despite losing.
Not the best ideas I ever rattled off the top of my head, but definitely the only ones I can come up with that I'd think would work.