I had a very similar custom card idea for another game, it had this effect "put a card from your hand face down in front of you and name it's cost.
During other players' turns they can swap it with a card in their hand of the same cost, if swaped both players put their new cards into their hand. At the start of your turn, if the face down
card is still there trash it and you may gain a card costing up to 2 more then the destroyed card and put it into your hand" These types of effects don't really fit dominion well in my book.
Voyageur- Forcing your opponent to trade without even this being an attack is weird. Doesn't work well with copper. Uses honor since it doesn't force hand reveals. I can see this card being extra swingly, you could force someone to give you a province late game if they're unlucky, a high value card early game if they only draw it with coppers, or you can get the same or a worst card. The only real value target for this card is estate, but even with the estate combo you could still get easily get a estate back. Also the card doesn't need the "discard this" text.