There's some interesting strategic space to explore, although balancing could get pretty silly pretty quickly.
Plantation
$6 Action-Duration
At the start of every turn, +$1
(The easy one! A bit better than Treasury, but I'm not sure I'd take Gold over it often, so maybe it should be 7?)
Missionary
$5 Action-Duration
At the start of every turn, trash a card from your hand or trash this.
(Optional Junk Dealer without the +$ but with a conditional topdeck, basically, so I figure 5 seems decently fair. I wonder how it would be without the option to trash it instead of your hand?)
Commercial District
$4 Action-Duration
At the start of every turn, +1 Buy
(Seemed more like a 3 until I thought more about Woodcutter and Market Square)
Con Man
$6 Action-Duration-Attack
At the start of every turn, each of your opponents gains a Copper.
(It could empty Curses too quickly to really balance, and Ruins seemed a bit too nasty as well, so I went for this. I based the cost off Mountebank, but I'm not too sure about it.)
Expert Craftsman
$6 Action-Duration
During each of your cleanup phases, draw 6 cards instead of 5.
(I wanted to avoid letting them stack, and wanted to give them a weakness to "Discard to X." Plus I got tired of start-of-turn. When I think about Lab I want to make it 7, but when I think of Caravan 6 seems more reasonable.)
Travelling Villagers
$10 Action-Duration
For the rest of the game, your action cards do not cost Actions.
(Ludicrously powerful, but is it worth building an engine around a $10 card? I could maybe even see pricing it at 12 or something, so maybe it's just plain too strong. I wish I didn't have to word this awkwardly to get around Diadem.)
Port
$4 Action-Duration
At the start of every turn, draw a card and then discard a card.
(Comparing it to Lab, it seems stronger than 4, but compared to Inn or Warehouse I'm not so sure.)
I'm sure you good people have better ideas, so let's see 'em!