Well, I was late on the draw, and I forgot to submit my entry in time. Whoops! But anyway I thought the entry I had in mind was unique and also useful, so I'll just post it here for critique if that's ok. I had some other entries in mind that I was eventually going to post here for non-contest related critique.
When you buy this or play it, choose twice: +$1; or trash a card from your hand.
Quicksilver would've been my entry into the Potion contest. It seemed like a great card to me, and I would pick it up even if it was by itself due to it being a non-terminal trasher and a non-terminal Silver (or something inbetween), so it would continue being useful once it has flushed all the cruft out of my deck. It doesn't explicitly say (it also doesn't explicitly say otherwise), but you can make the same choice twice.
You may discard a Potion from your hand. If you do, +$4. Otherwise, gain a Potion; put it into your hand.
Hello... don't you seem familiar? Maybe I should've named this one Vizier, which is also a title of nobility (like Baron) but I couldn't figure out a way to associate Potions with Viziers without it being weird. Oh well! Elixirs let you get Potions on demand, which can be quite useful if you don't immediately have a spare Potion to buy Potion cards (or in case you don't want to let your Alchemists go to discard). It's also less of a problem if you stock up on a bunch of Potions, since Elixir helps you convert them back into coin. Being non-terminal is probably a pretty big deal as well here (though the +Buy might've been more useful, considering the decisions Potion cards force on players).
Discard any number of Potion cards from your hand. +1 Card per Potion card discarded. If you discarded any Potion cards this way, +2 Cards.
In games using this, during your Action phase, you may discard 2 Potions from your hand. If you do, gain a Zeppelin; put it into your hand.
Zeppelin doesn't draw cards by itself, but it's got great mileage (if you've got the right fuel!). Basically, the first Potion you discard will be worth 3 Cards; every Potion after the first will get you one card further. Similar to Elixir, Zeppelins make it less of a problem if you end up going a little Potion crazy; if you've got 2 Potions, discard them and put a Zeppelin directly into your hand! And you can do this as much as you like! Now that's service.
+1 Card, +1 Action
You may trash this immediately. If you do, +1 Card.
When you buy this, +1 Buy.
Okay after seeing Rats today I'm going to figure that Homonculus is possibly undercosted here. It's a very simple cantrip, but you can choose to trash it immediately for a makeshift Laboratory. It also doesn't use up a Buy during your Buy phase, so even if you end up with a hand of CCCPE, you can still get a Homonculus and a Silver. All in all Homonculus doesn't do much but it's nice to have and isn't as strict a commitment as other Potion cards.
Liquid Gold $4P
Something similar showed up during the contest, but I'm going to conveniently ignore it. At its current cost, Liquid Gold is probably rather prohibitive, and with no other Potion cards in the supply it's probably not really worth it. Consider this though; when you draw Liquid Gold, all you need to play is a Copper to get more Liquid Gold. After a couple of turns, buying Liquid Gold will probably end up being pretty easy (maybe easier than buying Gold?).
Fountain of Youth $4P
Worth 1VP per 5 turns you’ve played this game.
Just from personal experience, my games tend to last around 18 to 20 turns, so $4P for around 4 or 5 VP at that point sounds fine. Maybe this is a little overcosted. Perfect for the games in which everyone's been beating each other up for awhile, or when everyone's been playing Ghost Ship each turn (that is, games that everyone hate). Extra turns from Outpost and Possession count towards VP for Fountains of Youth.