Feedback:
Christmas Chapel by majiponi
I can imagine this functioning very similarly to Chapel, except even more centralising. The effect for everyone seems tacked on and not really enough to make not going for this worthwhile.
Christmas Feast by pubby
Do the other players get to see the card before choosing to gain it or putting it back? Either would be interesting. I like this one a lot - it's swingy but in a fun way. I think it would be better as a workshop variant rather than an event, so there's an extra layer in deciding to "go for" the Christmas Feast deck, to make it more conscious than luck based. Still, I like the idea. Excellent response to the theme.
Snow Hill/Sleigh by grep
I imagine this being really good and fun in games where this is the only Village, but otherwise useless. The drawback is too much - compare to Lost City. You would want to not play sleighs to set up a chain later, but that actually nullifies the advantage of gaining to hand. Perhaps gaining to top of deck would be the best of both words. I like what you're going for here (one of my long time favourite things is everyone getting the same benefit), but I don't think this works quite well enough.
Oversee by Fragasnap
I think this is a bit too swingy. Estate->$5 card is very good in the opening (why Expand needs to cost $7), while in games with special cards it can backfire horribly. I don't think it's any more interesting for the trashing being mandatory,. The shared effect helps other people in an interesting way in Kingdoms with this card, giving some 2nd player advantage, but it all seems like through the lens of luck.
Midnight Ride by forkofnature
This card pretty much doesn't work sorry. A cantrip leaves you worse off than every other player for having played it. There are some niche scenarios where you are far better equipped to deal with a card than other players, but I don't think this card explores it well. Better luck next time
Santa Claus by segura
I like the Heirloom by theory/Donald X better, as I don't mind seeing a dead Heirloom as much as a dead kingdom card. Still this is an interesting card that makes the greening phase interesting and brings new life to gainers (in non alt vp games). The penalty might be a bit too much.
Santa's Workshop/Gift by [TP] Inferno
Workshop variants are on the weaker/situational side as far as cards go, so putting a penalty on one needs to have something to balance it out, and I don't think being non terminal with +buy is quite enough. I think this is a very good approach to the theme, and I like Gift, but I don't think the card as a whole adds enough interesting.
Magi by Gazbag
I really like the on gain penalty and am surprised there isn't an official card with it. This doesn't seem to fit neatly into any kind of deck I can imagine unfortunately. Engines aren't really that fond of Gold, it takes 3 shuffles for a single one to get the main benefit (not good for BM and Slogs), and I can't think of any good combos with it. Theme is nice.
The Grinch by grrgrrgrr
Very original. There are 13 imps so you will have 1 more than your opponents except in 5 player (if you're the first to buy). The best way to make good use of being flooded with Imps is a deck with good variety. The more I think about this, the more I like it.
Gift Exchange by Snowyowl
I like the theme of this, and it's nice and simple. In practice I think it will largely be "Gain a Gold, each other player trashes a card from their hand", but that sounds like a good card (I'm a fan of Leprechaun). There's a lot of potential for politics, but that sounds about right for Christmas.
Papa Noel by spineflu
Maybe I'm missing something but I don't really like the combination of effects here. It may be non terminal with Capitalism, but in most games it is terminal. Effects for other players are basically "+1 Card, +1 Action, (effect)", but with no opportunity cost. The opportunity cost for you on the other hand, is big. I like the Victory -> VP effect but it's not really carrying the card. I'm also not getting a lot of theme.
Sub for Santa by mail-mi
Thinking about theme here (Christmas), this card can be way too mean; gain a $2 with it and it's basically a Mountebank. I think it would be better if the Curse and other card were optional (gain a curse to gain the card - I had a fan card that did something similar). The 5/4 gap is very big and I think that makes this card too good.
Ghosts of Christmas by somekindoftony
While I did say I focused more on theme I was hoping that each card would satisfy the competition criteria (something positive for your opponents), and this does no such thing.
Looking at theme, I think what you're going for here is that the Ghost of Christmas Future scares you but gives you another chance at Christmas. This is a nice idea and the cards work well with it. I think it can best be compared with a Journey token card as an interesting spin on chapel which plays a bit with the early game (do you really want to use your oneshot gold now?)
I think on theme alone you'd be competitive, but it would be better with some sense of the idea of "giving" in the theme (even if you wanted to stick with "A Christmas Carol")
Winter Wonderland by scolapasta
I like it. You'll probably want to pick one up but you can make not picking one up work well for you too. Late game I think it mitigates greening a bit too well and the +VP may not really be necessary.
Magic Workshop by Something_Smart
This is very strong, and I think there are far too many situations where you can come up with a card other people don't want.
Christmas and a whole lot of other things, by anordinaryman
I asked people to go hard on theme, and nobody went harder than you.
I really like "Naughty" although to make it work as intended, I think a better every player should start with "Nice", with "flip this to Naughty when you play an Attack" written on it. Maybe something obscure but not unavoidable like "Trash an attack" would be good for getting back to Nice (which is an important part of Christmas IMO - you aren't naughty forever, forgiveness is possible).
Probably the best aspect of this card is that it works well when everyone chooses to be nice. Once someone's mean, you can flood them with coal.
Only issue is that there might not be $4 cards in the kingdom you want to gain a lot of. This would ultimately benefit the "naughty" player who is happy to get $4 Labs, and you end up with coal flying everywhere.
Still, this was the sort of thing I had more in mind when I came up with the competition.
RESULTS
Winner - Gift Exchange by snowyowl
Runner Up - The Grinch by grrgrrgrr