All of the above, but also organized around a Durlag's style dungeon crawl for the anti-climax.
I think the idea of a magical tower or secondary hub area where the mod content could be introduced. Candidates might be Hag witchcraft the swamp mushroom circles, the sentient Mushrooms in the underdark, or via the circus in Act III. I mean like if we were going to drop an NPC somewhere that could take us to the tower, those feel like they'd be good spots to re-access the earlier areas. Like the Djinni could definitely do some more Jurassic Park stunts to teleport us back around to the locations from act I. Or via the portals at the sundries or the mirrors in hell, where we could transition back for a big Dungeon Tower crawl.
Another thing would be some form of swimming or ropes to allow us to access places we can see but can't reach normally, and to place transition there. Like the little hidden heights or outcrops that are more challenging to reach, or feather falls that replace chasms with actual mini areas. That might be a challenge right there, just anywhere that we can chasm to death, but instead of dying we wind up in the new spot. Like say we get yeeted into the river, to show up somewhere downstream in an area that branches off from the main locales. First move should be jump into the river by the crash site, and you end up in Modsville. That would be fun hehe
