Here is my submission
Feedback welcome
I think it need to say "for the rest of the game".
Updated original entry now does
+1 Card
+1 Action
+1 Buy
At the start of each of your turns for the rest of the game, you may trash 2 Coppers to gain a Silver or trash 2 silvers to gain a gold. These are gained to your hand
(This stays in Play)
Very cool idea for a submission. Per your request, here is some feedback, both technical/copy editing and substantive:
The card type (at the bottom) should be "Action - Duration" not just Action.
The phrase "This stays in play" should have a period at the end, inside of the parentheses, and the P in play should be lower case (see Hireling).
In the text ("trash 2 silvers to gain a gold"), the words "silvers" and "gold" should be capitalized. Also, it needs to specify that the 2 Coppers or 2 Silvers are being trashed from your hand (see Mine, Moneylender).
I think the last sentence ("These are gained to your hand") is a bit ambiguous, and the total text would not be longer if you added "to your hand" after both "gain a Silver" and "gain a Gold". If you are going to leave it as is, it needs a period.
Substantively, I think the card might be too powerful. The Copper to Silver swap is a no-brainer, and accomplishes something that's an incredibly important thing to do (getting rid of your Coppers) any cost to any turn. Usually, the turn on which you trash an item is somehow lessened by the trashing. The Silver-to-Gold has some cost, since the value goes from $2 to $3, but given that it is an option every time you have two Silvers in hand for the rest of the game, it's still an incredibly useful tool.
At the very least, I would suggest getting rid of the Vanilla bonuses (+Card/Action/Buy). If you are getting a bonus for the rest of the game, it is not at all unreasonable to make you use a terminal action on one turn to do it. Plus, they're taking up half the cards space and shrinking your text way down (which is going to get even smaller when you add "from your hand" twice).
Hope that was clear/helpful.