Aeon's End: War Eternal
Buy this firstBGG and Reddit buyers say to get a standalone big box before any small box; War Eternal is the pick for harder nemesis fights, though one BGG poster calls it a bit too punitive.
Needs: Standalone: playable on its own and fully compatible with the original game; also usable as the base box for small-box expansions.
Adds: A complete second big box: 8 mages, 4 nemeses, 27 supply sets and 25 basic nemesis cards, with harder nemesis fights.
Fixes: Gives a first-time owner more mages, nemeses and market variety than the base box alone, with tougher fights than the original nemeses.