Id rather just say its Classic +, not WoW Classic the Hollywood Reboot.
I can only tell you how I would do it if it was up to me so take it for what its worth, which isnt much. Listen up Blizz.
For starters, Id take a peak at what was left on the cutting room floor from original vanilla and see whats viable there and doesnt conflict with any content seen in later expacs. For example, I wouldnt put Kara the instance in there although something different could be done with that area - maybe an open world dungeon (like a Jintha Alor) instead of a raid but again, ideas are limitless. Do something with that underground dungeon there that was never finished, I dunno.
From there I would take steps to make sure content couldnt be trivialized - so no world buff metas or dungeon boosting. Bug fixes and tweaks. Old school AV, no cross server queues. Make improvements to quest reward gear (the NPC will always have something your class can use). Introduce better itemized sets for less popular specs and just slap them on the bosses loot tables. Flesh out any unfinished zones and/or quest lines. Add that BG we never got.
Basically, I would build on whats already there without drastically altering the entire landscape. Changes to raid size, raid encounters and an increase in raid boss drops are not off the table to me.
From there, instead of introducing new story arcs and the like I would simply have GMs run newly created events that stay within the context of the current game world - Like Northwatch (from the guns of northwatch quest) is, for a weekend, a level 55-60 area that requires the defeat of a few elite mobs and completion ends with a nice blue reward - maybe a choice of new gear enchants or new patterns. It will have both solo and group quests blah blah. Thats just one tiny example.
Im also a big fan of dynamic spontaneous content like a new world boss spawning randomly somewhere - or the barrens centaur are tired of being stomped by horde lowbies and now theyre raiding crossroads for an hour to get revenge.
Add a new profession.
I could go on and on but I think you get the idea.
Edit: Just wanted to add I would overhaul the honor system and add more ranks and gear. Id also put Wpvp objectives in every zone and rework existing ones.
Edit2: Ive always had my opinions about how I wouldve done arenas in WoW and Id try to get a version of that in - but there wouldnt be any kind of official ladder or rating.