Fortune Seekers$2
Action - DurationThe player to your left chooses one for you to receive: +2 Cards; +1 Card and +1 Action; +$2.
If you have at least 2 other copies of this in play, trash this and gain a non-Victory card to your hand. Otherwise, at the start of your next turn, +1 Action and +1 Buy.
Notes: An upgrade of Treasure Map designed to be less swingy in a number of ways. First, it takes advantage of the Duration mechanic to require you to draw three copies in two turns rather than two copies in one turn. Second, it's not dead when you don't upgrade it, just a bit weak. Finally, it only lets you trash one copy for one reward at a time. This last one wasn't entirely intentional - having it be a Duration meant I couldn't trash the copies already in play without creating tracking issues (which already happens, but seemed too difficult for an early expansion like Seaside) or using awkward language like "At the start of your next turn, if this is still in play..."
The on-play effect is designed to mimic "following the map." While the options may look strong (and any one of them would likely make the card too strong if it was a fixed effect instead of at the mercy of your opponent), your opponent should usually be able to find something that isn't that helpful---there's a reason Advisor is much weaker than Laboratory. Earlier versions were a Command "Your opponent chooses a non-Command Action card from the Supply for you to play, leaving it there," which was too swingy and potentially complex, and a Pawn chosen by your opponent, but then I decided to narrow it to the three generic benefits that cards often get in addition to their more unique effects: minimal terminal draw, cantrip, and terminal Silver.
The +1 Action is to guarantee that it's never impossible to get three copies in play (barring edge cases like Black Market): If nothing else you can play one copy on one turn and two on the next. The +1 Buy is to make it easier to buy more Fortune Seekers, and because Seaside generally has too many cheap cards and not enough +Buys for them (just Scavenger, which doesn't increase your deck size, Tactician, which normally doesn't net you buys because it costs you the ability to buy anything useful the turn you play it, Wharf, and arguably Outpost). The fact that it can gain cards other than Gold is just for fun, flexibility, and uniqueness.
As a bonus, here's a tokenless Embargo-Haven mashup that I didn't think was original enough to use as an entry but wanted to share anyway:
Monopoly$3
Action - Attack - Duration+$2
Set aside a card from your hand face-up. At the start of your next turn, put it into your hand.
When any other player buys a copy of the card set aside here, they gain a Curse.