Another thread about some overpowered class with very limited use so a few can murder everyone in the battleground? no thanks
Tyranids should be the only game changer. Adding the LotD or Harlequins would favor only the Space Marines or the Eldar.
Holiday events could happen, but something about your proposal rubs me the wrong way. Maybe it's that these units will be a limited time only thing, and that everyone will rush Harlequin/Grey Knight/whatever on those holidays to try them out.
Please no holiday events. And please, don't include those special snowflake guys, because if you do it, people will start whining about how they can't play their SUPERSPECIALEDGY character. It's bad enough as it is. On a second thought, there is one special character you could include: The Easter Carnifex.
It would have to be the Tyranids or a neutral race if you're going to implement a huge NPC threat on the battlefield because like people said, putting the Legion of the Damned into the game would only benefit 1 of the 4 factions. 3 of the factions would just hate it So the Devs would have to put in an additional 3 "special" units or factions for the other races in order to make it even and that's a little too much work for something like this
Just put in some "Special" free holiday cosmetics for people's characters and I think we'll be all set.
The first thing that came to my mind when I read the poll (but not the OP) was "A Harlequin troupe performing!" which would be something special and I imagine pretty nice. Problem is, I don't think there is something along those lines for the other factions, is there? As playable classes, I still strongly oppose them. Also I'm sort of in favor for not actually having holiday-themed skins or something... as long as it doesn't fit the universe (which is is a pretty big hindrance, me thinks)
It would be fun but shouldn't be put in the hands of the players. Just have occasions where the devs pop up as a band of dark eldar marauders or some such thing or a bunch of rampaging carnifexes. It'd be a great way of occasionally mixing stuff up, and you wouldn't have players bringing faction politics into it.
There's always faction politics. If the dark eldar attack someone, that faction will gripe that they've got developers kicking them. If the devs appear in a spot and say "Hey Crusaders, come and kill us for a shiny bonus" then someone will gripe that its right near SM territory and of course the SMs had wiped the floor and got the bonus long before anyone else arrived... They would be better as rare and random spawning NPCs than any situation where a developer walks into the game's campaign map as a developer (not a regular player), picks up an OP class that only they can use and proceeds to blow the heck of out of players. For everyone who'll think that's kinda cool, there's going to be a lot of people that got churned up into corpses who'll be annoyed about it.