#1 Shinsuke Nakamura and AJ Styles

As much as it pains me to write this, this option shouldn't be ruled out. The WWE teased the idea of having a triple threat for the WWE title at last year's WrestleMania, with AJ Styles desperately trying to force his way into the title match between Randy Orton and Bray Wyatt.
The WWE is not one to shy away from repeating previous storylines at this time of the year, and Randy could easily ask Shane to repay his loyalty and insert him into the WWE Title match at WrestleMania 34.
Of course, Randy would have to earn that shot and a match against Shinsuke Nakamura at Fastlane seems like exactly the kind of bout the pay-per-view is looking for.
There would be a huge fan backlash to this decision but does it really seem that implausible that Vince McMahon would want to put a 'WWE guy' in a match at WrestleMania against two superstars who made a name for themselves elsewhere.
Having a WWE Title match at WrestleMania between two 'non-WWE/New Japan' superstars does seem like a bold move from Vince McMahon, so don't rule out him getting cold feet at the last moment and putting Randy Orton in the bout as well.