I am not too happy with the current version. Not so much because the debt is not elegant but because it doesn't punish multiple types harsh enough. The debt version with 3-2 could just be too easy with gainers so I am going to try a straightforward cost increase. It shouldn't run into any issues except for trashing attacks. Unless you have an Action-Duration in play Knight (at a price of 7 it is safe) and Swindler (unless there are others 7s you get another Gate) could kill this.
In Adventures, we tested a Treasure that was worth $1 per different type on cards you had in play. It varied too wildly in value, so it turned into a card that cared about having specific types in play, and then finally vastly simplified into Relic.
My suggested fix to the original concept was to also vary the cost, much like Gate; $1 per different type on cards in the Supply. Donald X. never tried it, but in theory it seems like it should work pretty well. I also slapped +1 Buy on it so that it was worth getting over Silver in games with just Action, Victory, Treasure, and Curse in the Supply.
Curio: Treasure, $?
+1 Buy
When you play this, it's worth $1 per type on cards you have in play.
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This costs $1 per type on cards in the Supply.
So if you had, say, Caravan (Action-Duration) and Militia (Action-Attack) in the Supply, Curio would start out costing $6 and you could get it up to being worth +$4. The cost would go down to $5 if the Caravans ran out, though. Just as an example.
Anyway, it seemed like it might be fun to try in a game, though I don't know if would actually have lasting appeal. Courtier has already used this concept a bit, and in an arguably more interesting way.
Sorry that this post didn't have anything to do with Gate. It just reminded me of this story.