At this point, unless Orton wins the Rumble or unless Brock signs an extension and they fight later on (without the title on the line, but I'll take what I can get), looks like WWE missed out big time on a potentially great rivalry. Orton's Viper persona, especially when he's at his most deranged, would have made a great adversary for Lesnar, and Brock conquering a multi-time world champion/Rumble winner/Mania headliner/etc. would have added a lot of meat to his title reign and his list of accolades.
On top of that, the match would have been completely fresh, having only wrestled each other once throughout their entire careers and that was just a short-ass match back in 2002 when neither man was even a big star yet.
Orton hitting the RKO on Lesnar out of nowhere, Orton hitting Brock with the rope-hung DDT, Orton punt-kicking Brock only for Brock to kick out at 2 (and then maybe delivering a punt-kick of his own to Randy), etc. The match and the whole feud could have been excellent. I was re-watching some of Orton's old Viper stuff from 2009 during his feud with Triple H and was reminded just how much his character was on-fire in those days.
Even though calculating the odds I know it logically won't happen, I still have hopes that maybe Lesnar will work Fast Lane against Orton.