There are a few options for Classic endgame. None of them involve Classic+ so this idea needs to be jettisoned from the community vernacular and nuked in orbit. It will never happen. Ever.
So let's talk Pro's and Con's of the options that will happen:
Scenario A: Nothing
Blizzard lets Classic realms stay on the Naxx patch forever. No new content other than updates about Holidays and shit like that.
Pros: By far the easiest and least maintenance solution for Blizzard.
"Real" Classic fans will love it.
Cons: This is also the least sustainable solution. After awhile, servers will begin to die out. At some point they'll either need to merge/shut down servers.
There really won't be a very good path to endgame for new 60s. You will either join an established guild and get your ass carried or you'll try to muddle your way through lower endgame content with the 3 or 4 other unlucky souls who don't yet have a Naxx raiding guild.
"Real" Classic fans will hate it.
Scenario B: Nothing, but with Seasonal servers
Blizzard introduces "Seasonal" servers which give players a chance to experience the many Phases of Classic again with all the sweet knowledge they've already obtained.
Pros: Keeps the game interesting for people who like Classic but don't like the idea of staying on the same dead/dying realm slaying the same internet dragons until the end of time.
"Real" Classic fans will love it.
Cons: There's no guarantee people will roll on Seasonal realms. It also could potentially majorly fuck with guilds if their players are split on whether to reroll.
"Real" Classic fans will hate it.
Scenario C: TBC because fuck you
Blizzard announces TBC Classic and says fuck you, you're upgrading to it whether you like it or not.
Pros: Easiest and least maintenance intensive way forward for Blizzard to "continue" Classic. It doesn't risk splitting the community because fuck you, you've got TBC whether you like it or not.
"Real" Classic fans will love it.
Cons: While this doesn't risk splitting the playerbase, it does risk alienating the few weirdos who would prefer Scenario A. It also kind of puts Blizzard back in the weird position where there's no longer a legal way to replicate the aMaZiNg ClAsSiC eXpErIeNcE.
"Real" Classic fans will hate it.
Scenario D: Classic realms AND TBC realms
Blizzard decides to pull a Dora the Explorer and says fuck it, why not both? Blizzard adds a metric fuckton of TBC servers and you character copy your happy ass over to it or reroll and deal with it.
Pros: Satisfies both the Classic fans and the TBC fans.
"Real" Classic fans will love it.
Cons: Splits the playerbase. Some Classic realms will definitely die. And dear God, the character creation process will be a fucking nightmare as faction balance is totally skewed and everybody rolls on PvP servers despite hating everything about world PvP "because that's what my friends like."
"Real" Classic fans hate it.
Scenario E: Seasonal realms, TBC realms AND Classic realms
Blizzard goes full madman and does it all.
Pros: If you can't find a version of Classic to play, you're either too picky or you frequently post on this subforum.
"Real" Classic fans will love it.
Cons: Logistical nightmare for Blizzard and will definitely fracture the playerbase, leading to inevitable server mergers/closures. It also runs the risk of trying to accomplish too many things and ultimately appeasing nobody.
"Real" Classic fans will hate it.
Phew. That ended up being longer than I expected. If I were a betting man, I'd say only Scenario A and C are probable. But who knows. Only time will tell. Oh, and no matter what Blizzard does or doesn't do, "real" fans will either love or hate them for it.