The smithy card in dominion would use 3-5 cards and would be modeled in the following way:
 [Action] [→]  [Draw Card]
For thematic purposes I am probably going to make the effects that you build be named "engines", and the game will probably be themed as a magitech game where you are building perpetual motion machines or something.
I have been playing around with rules and came up with a first attempt set of rules with Alison. I refined the initial rules over a few days until I felt that it was likely that a first run through of the game could be made.
Last night we played our first game and it went pretty well. Alison built a fancy engine to convert Time into Prestige and she won via a flood of prestige.
We didn't like the fact that it was so hard to make engines, so we modified the rules and played another game of it tonight. We still ran into some trouble, but the game was a lot smoother. Hopefully a few more modifications will make it feel smoother still.
To sum up tonight's game, I built a perpetual motion machine that got me an infinite amount of Justice and then used it to build a terribly unbalanced machine that converted Justice into Prestige and then won the next turn with infinite Prestige.
One option I am considering is a 'go fish' type mechanic where you can name a card and then if it is in any players hand they give it to you in exchange for one of your cards.
Either way, it is a pretty fun game to make.