As @winsens says WoW Forever is basically Vanilla+ or Classic+ there are a mountain of changes but a very top level view is the world will never go to BC, it will remain on Azeroth forever, 60 cap forever, new raids, new dungeons, changed talent trees, some new spells, new areas, new stories, it is a complete divergent timeline vs the OG vanilla>tbc>wrath>etc.

One thing I am curious about Longer term is the devs only said staying on Azeroth, which means Nothrend is still on the table in my mind, or it could be, as are many continents retail wow unlocked over the years.

How close is it to Classic? Id say closer to classic than TBC or wrath, even tho several changes from those expansions have made their way into Forever, like unified hit for melee/spells, and a host of other tweaks. https://wowguide.net/en/guides/what-is-wow-forever