This is more in line with what I'd expect out of an expansion.
Pretty much similar to the content that current mods provide with more world bosses or mini zones for the end-game, and a few new crafting mechanics like the Worldstone Shards in ProjectD2 which adds a gambling component to augmenting items. I think that'd spice it up nicely, and as an expansion be completely optional to those who choose to play with this modification in mind.
I think a best-case scenario could be completely incorporating a mod into the game as an official 'paid DLC' expansion. Exactly what the new Binding of Isaac: Repentance pack will be doing by incorporating the fan-made Antibirth mod.