Here's my idea for an expansion. The theme is a paradoxical combination of Prosperity and Dark Ages: basically, blowing all your resources at once for massive benefit. It has a lot of cards that trash themselves, and also some overpay mechanics. It also has a new type of card called Luxury. Luxuries are similar to Spoils in that they can only be gained with other cards, but they don't enter your discard immediately. Instead, you can put them in your discard at any time, so you can save them up and then blow them all later on. Also, they trash themselves instead of returning to the pile, motivating you to save them. I may make other Luxuries, but this is main one I have now:
Parade
$0
Action-Luxury
+3 cards
+1 action
Now onto the cards:
Gold Vein
$5
Treasure
Worth $2
When you play this, gain a Parade.
My idea for a treasure that draws.
Souvenir
$6
Action
+1 card
+1 action
+1 coin
When you buy this, you may overpay for it. For each $1 you overpaid, gain a Souvenir.
Wait until you have a ton of $, then get a bunch of these with the overpay.
Accountant
$1
Action
+2 cards
+1 action
Discard 2 cards. You may trash this and your hand.
When you buy this, you may overpay for. Gain a card costing exactly $2 more than the amount overpaid.
My idea for a better Bomb. The overpay lets you get a card for $1 less as well as a cheap sifter/trasher that you might or might not want.
Crown Jewels
$4
Treasure-Victory
Worth $1 and 1 VP
When you buy this, you may overpay for it. +1 VP per $1 overpaid.
The more you overpay for it, the more VP tokens you get.
Goldsmith
$5
Action
+3 cards
You may discard 3 cards, to gain a Parade.
When you buy this, you may overpay for it. For each $1 overpaid, gain a Parade.
If you have some junk after drawing, you can discard it to get a delayed benefit.
I'll post some more later. For now, what do you think of these?