What simple cards do you think are left that haven't yet been done? Here's some I have.
Viking Village - $4
+1 Card
+2 Actions
Gain a silver.
Although simple, I think Viking Village is interesting in that the engine that you can make with it gets clogged up with silvers, which can be good if the engine is strong enough to draw your deck, but also has the potential to make your engine very hard to start.
Farmer - $2
+1 Card
+$1
You may play a Farmer from your hand.
This is a simple peddler variant that's good to get in multiples. The variation part makes it worse in engines, and peddlers aren't that good in big money, so it's an interesting decision about when to buy.