Probably doesn't hurt to start thinking early. And maybe getting a bunch of notes we've collected over the years in one place will jog something. In 2016, Kinak said:
Guess I should mention that there are two new abominations to find, on top of at least that many undiscovered from previous years.
That's the year we found the pyrotechnic snowman, but we didn't find the other new one. Since the manyheaded snowman appears to have been an addition new to last year, that means we've still got three undiscovered snowmen (at least one of which is presumably a snow snake), plus figuring out how to properly encounter the manyheaded one.
Looking at item IDs, we have potentially missing drops from 2011, 2012, 2013, 2014, 2016 and 2017. Some of those gaps are in places that it's hard to say if it's Christmas content or just stuff that was released near then, though.
Looking at the various hints we've gotten on snakes, I wonder… Can anyone confirm that they have killed at least some punks on the quad during Christmas events in past years? There'd be a stronger link if the snakes were added the same year the Punk Uprising quest was implemented, but from stuff below it does look like the snakes were added in 2013. Still, because of the way the uprising quest works, we know punk deaths is a thing that's being counted, at least on a per player basis, so it would fit in with some of the hints below.
Hints on the snakes (and a bit on other missing snowmen) -
2014:
Q: Any hints at mysterious snake snowmen we seem to have missed this year? Or will we just have to try again next time?
A: They will be there next year. Kind of funny. I was planning on having an arc of them but things got well, 'hey there’s things going on here' but not til the first actual unlock. But I was planning to have more stuff past there but didn’t end up getting to it because of the server thing.
2015:
Q: It looks like again last Christmas we didn't make much progress toward figuring out the snow snakes. Any chance of a hint to keep in mind for this year?
A: I actually tried to change the description to make that a little more cluey but didn't help. I think I will try to add more clues next year.
In 2014, the message for the shadows appearing had changed from 'The snowmen's shadows carve strange patterns on the snow, as though tiny snakes had been dancing over the drifts.' to 'Everyone's snowmen cast strange shadows, carving patterns through the snow like tiny snakes had been dancing over the drifts.'
2017:
Q: I found it curious you should mention during Christmas that you only liked adding content for currently available unearthly items. Was one of those hidden snowdudes related to an unearthly? Or any of that new Christmas content unearthly related?
A: I think the reason that came up, someone was asking about wearing the knitted hat with sweaters, and I didn't want to add that because the hat was not in season. None of the new snowmen were unearthly related. Something I hadn't been counting before I started counting, and the petrol stuff, and some old stuff people haven't figured out.
Q: It's still a few months away, but any chance of some clues on finding snow snakes this Christmas?
A: Um. Yeah! It's sort of, if I recall, going a little orthogonal to the normal thrust of the things you are counting at Christmas. Not a new thing that is being counted, just a different thing that is being counted, and just hasn't gotten to the levels needed. I have full faith you'll get there.
Also, a note I recorded monitoring the leaderboards that year. At the point the patterns appeared, we seemed to have -
According to the screen, there were a total of 228 snowmen built and 300 torn back down. The total Abomonometer reading is 557.
An Abomonometer reading of 500 had not been enough to trigger it. And it was probably at around 290-something torn down when I saw them.
2018:
Q: When helping out the Hipsters, sometimes you build monstrous snowmen and sometimes you build tiny snowmen. Is there any way to control that?
A: [Searches] Those encounters are more indicators than something you need to… not counting x number of small snowmen or large snowmen. Letting you know when flags go up.
Q: Any hints on snow snakes? I'd really like to actually see them one of these years.
A: It's just counting something that's fron a direction that's a little different from the rest of the snowmen, and there's rewards off the beaten path and that's why they haven't been encountered yet.
2019:
Q: At the start of last Christmas, the Manyheaded Snowman accidentally showed up in the Snow Goons encounter. Were we ever close to unlocking it for real?
A: (checks) There's some crazy stuff in here. Oh, you were 4% away from unlocking it. You got super close. You'll get it next year with a little bigger push.
Q: Speaking of Christmas, we've fought the Abominable Remains only once on Christmas 2010 and the Snowmonstrosity from Christmas 2011 on. Would it be possible to get them to alternate, or to get some kind of choice encounter where people would be able to influence which one shows up?
A: Hm. Yeah, I should either make a third boss, or bring that back. The Abominable Remains are pretty awesome.
Keep an eye out for a different boss this year?
Q: Christmas followup! Any way to influence if we build tiny or huge snowmen from the hipsters?
A: I believe that is based off a communal counter, it switches over after a while. Nothing that an individual can do, other than advancing stuff.