I got into a discussion about this card in one of the treasure chest threads, so I figured this is a topic worth resurrecting. Maybe 2/3 of the people here say the optimal strategy is to toss a copper every turn, unless it's the difference between $7 and $8 in the endgame, while the other third say that you shouldn't bother, and just let the curses in and treat it like a witch. I'm in the minority (it's worse than Cutpursing yourself every turn, and sounds almost as bad as always discarding to Torturer), which, if true, would just make this a $4 Witch. Even if the choice does turn out to be obvious, with more playtesting or simulation, it's inherently unfun. We already have Torturer, and it fills that 'much-needed gap' well. Compared to the other $4 cursers (+nothing to you, or +2-2 cards, guaranteed Moat on the board that you probably want anyway) it seems way out of line.
I should actually playtest this before shooting my mouth off, but I wouldn't choose the discard unless I knew it would solve my problem and I wasn't making an open-ended commitment. My first thought was to scale the penalty with the number of curses, but cards on the Soothsayer mat trash themselves after sitting there N turns. I have no idea how to state that on a card, and if I did there would literally be no space for it.
You guys know what happens if people post here without mentioning the edge cases, so I said I'd also discard if it wasn't a hassle and I could make those curses miss the reshuffle. If that turns out to be the appropriate counter to a Soothsayer, we could refocus the card around that.
First draft, didn't change anything above the line:
"At the start of your Buy phase, if you have a Curse on
your Soothsayer mat, you may discard a treasure from
your hand. If you do, discard and reshuffle your deck.
Discard all cards on your Soothsayer mat."
Here, now the cursing is a given, but the effect can be delayed and the victim gets a free reshuffle. If this is still too weak, you could add a +Buy or +$ up top (to work the Loan Shark angle?) I'm worried that its schtick overlaps with the official Soothsayer card, but it's probably a different enough idea.
EDIT: Oh I see why you need the 'in games using this' clause. Maybe it's not necessary now.
Second draft:
"+2 Cards
Every other player may discard a Silver[?] from their hand. If they do, they discard and reshuffle their deck.
Every other player gains a Curse."