Get Hellfire. I did, and it rocks. Not only you get 8 new levels in two types (Nest and Demon Crypt, entered from the town), a bucketload of new uniques and a few extra prefixes/suffixes and a horde of monsters (though some just re-use obsolete animation files that were left in the game by Blizzard) as well as three new characters (Monk plus two hidden ones - Bard and Barbarian) but you can also select difficulty level like in multiplayer.
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