#4 Sting
'The Icon' Sting is one of the greatest professional wrestlers of all-time. Sting was to WCW what The Undertaker is to WWE and rightfully so, would be a 6-time WCW World Heavyweight Champion and hold multiple other titles before WCW closed its doors in 2001.
Sting would go onto wrestle a few matches for the World Wrestling All-Stars and TNA in 2003, before seemingly having possibly retired. Sting shocked the wrestling world in late 2005 when it was announced that he had struck a deal with TNA Wrestling to perform for them on a part-time basis in 2006, a relationship that would last until early 2014. During his time with TNA, Sting would become a 4 time TNA World Champion and would also hold the NWA World Championship once more and the TNA Tag Team Championship.
For over 13 years, it really seemed like the Stinger would always be that one huge name who would never work for WWE. That all changed at the 2014 WWE Survivor Series when Sting made his WWE debut. During his time as an active performer in WWE, Sting would compete in four matches, with his final match being a WWE Championship match with Seth Rollins at Night of Champions 2015. So really, it seems almost impossible that Sting could have held a Championship. Almost impossible, but not entirely impossible.
I really don't think it would have been impossible for Sting to be a Champion in WWE, especially when you consider things like Bret Hart winning the WWE United States Championship from The Miz in 2010.