Don't see how Orton could be viewed as a credible threat to the streak by the way he has been booked even though he is the champion. Only way he could be taken seriously is if he were to completely revert to the Viper/Predator type character and be booked super strong until WM31.
That being said, I don't have any interest in seeing Randy Orton challenge the Streak again. His time for that is gone.
My preferred options to challenge the Streak are:
Brock Lesnar
John Cena
Sting
the Rock
Those 4 are my picks. Lesnar seems to be all but confirmed for this year. I assume that WWE, too, see the hype and money in a Taker vs Cena match, but at the same time, they probably sit hoping that the Undertaker can keep good conditioning until 25-0(in the ideal world).
However, based on the reports about Taker's condition, him not being able to make it to 25-0, might well mean that the Streak might well be broken within the next couple of years,lMO.