From a Classic and Retail player, I like both games, but I also dislike some things in both. Things like transmog/pet battles/heroic dungeons all appear good to me. Things like power creep, 15 levels of the same piece of gear due to the 50+ levels of Mythic+(exaggeration), just feels like bloated content that shouldn't be there.
Then you have Classic where the content can be really hard to near impossible solo, but thrives in a group setting. Classic builds social communities good and bad, but the content is limited and can get old really quickly.
I think if we pull together great elements from both and apply them into an entirely new and separate game, i.e. WoW2/Diablo:Santuary/Starcraft:Universe(All MMO ideas) it would be an amazing game. I think this is the only way to put in a Classic mentality and game structure into retail. I think the current WoW timeline is too far gone that there is zero way to bring it back to Vanilla.
Let me know what you think.