on their own:
Vault
Death Cart
Chancellor
Scavenger
Baron
almost on their own:
Develop + any$4 + any$5
Poor House + any terminal silver
Masterpiece + any $2 card that makes money
combo's
(secret chamber/storeroom) + draw3 + village
cellar + highway + feast
warehouse + remodel + $4cost
nomad camp + fool's gold
baker + lots of things
stonemason + coppersmith
horse traders + menagerie
harvest + duchess
Contraband + crossroads
Butcher + crossroads
Mandarin + contraband/cache/counterfeit
beggar + Farmland + any$4
Counterfeit + Fool's Gold
Counterfeit + Courtyard
Graverobber + Crossroads + any$3
Pillage + Cellar
I think I'd ban these:
Vault, Death Cart, Chancellor, Scavenger, Baron, Develop, Poor House, Masterpiece, Baker
Secret Chamber, Storeroom, Farmland, Horse Traders, Stonemason
Cellar, Crossroads, Warehouse, Courtyard
Harvest, Mandarin, Fool's Gold
which brings my current estimation to 21
I don't think you can just remove any of the cards without breaking the puzzle.
Off course maybe you can remove 2 and add 1, and it's very likely some cards still need to be added.
I'm a little worried still about Butcher and Graverobber, but with these bans I don't see how they could work.