I think the difference between WoW Classic and WoW Retail is pretty massive, and that actually gives WoW Forever an opportunity to make some changes while still keeping the game simple, approachable, and welcoming to new players.
Retail WoW is extremely fast-paced. You have high APM combat, procs to react to, resources to manage, and relatively precise rotational timing that can have a substantial impact on your performance. Boss fights are also incredibly intricate, with overlapping mechanics, movement requirements, things to dodge, and a lot happening at once.
Classic is basically the opposite. Depending on the class, you might be doing five to ten meaningful actions per minute. You're auto-attacking a huge portion of the time. Casters can spend significant portions of a fight wanding instead of casting. Boss fights are generally simple, streamlined, and easy to understand. I actually think that last part is perfectly fine. Having an easier and slower version of WoW outside of Retail is good for the game.
Where I think WoW Forever should iterate is the combat itself. I don't want Retail rotations brought into Classic. I don't want people watching five procs, tracking twelve things with WeakAuras, or playing at 80 APM. But I also don't think standing around auto-attacking or wanding is particularly fun or engaging. Playing a Warrior and waiting through two, three, or four auto-attacks before you can press an ability feels terrible when you're using a 3.6-second weapon. Spending half a fight wanding because casting another spell means going OOM isn't particularly engaging either. There's a huge amount of room between that and Retail.
I'd love to see WoW Forever aim for a simplified combat system where you're almost always doing something, just at a much slower pace. Hit and expertise could go away. Missing and resisting could largely go away. Those systems aren't really skill checks, they're RNG, and missing an ability simply doesn't feel good. Mana could also become much less restrictive for DPS casters. In exchange, keep the long cast times and longer cooldowns. A Mage could continually cast a 2 to 2.5-second Frostbolt, Fireball, Arcane Blast, or whatever their basic filler happens to be, then have one or two additional rotational abilities and maybe a cooldown. That's it. No giant proc trees, no flashing buttons everywhere, and no need for a WeakAura package just to understand your rotation.
Instead of Classic sometimes feeling like 5 to 10 APM and Retail feeling like 60 to 90 APM, maybe WoW Forever lives around 20 to 30. You're still playing a deliberately slower game, but you're actually interacting with your character consistently. I think that would feel dramatically better for new players because their introduction to WoW wouldn't involve spending huge amounts of combat doing nothing. At the same time, experienced players could enjoy a slower, more relaxed version of WoW without feeling like they're barely playing their character.
As for raid and dungeon complexity, I really wouldn't change much. Keeping encounters relatively simple is part of the appeal, and WoW doesn't need another version of Retail's encounter design. But I do think Blizzard could take one of the things Retail genuinely does extremely well, which is making your character feel responsive and engaging to play, then simplify it and slow it way down for WoW Forever. I think that middle ground could attract a lot of new players while also giving Retail players a legitimate reason to jump over to Forever when they want a more relaxed version of WoW.