I think it's more likely to be Bryan-Orton as apparently the former hasn't been the latter clean? Don't know if that's right.
Orton will probably spend the couple of months prior to Mania facing Batista before facing Bryan at Mania.
Cena will probably get the Undertaker gig.
WWE still love Cena and doubt they'd want him to lose to Bryan clean unless they are 100%, one billion percent confident in Bryan as their main man and universally behind him and I mean that as in backstage. If not, then they'll do Orton-Bryan for the belts.