Hmm, good question. Plushies are always good, as is seasonal food and drink. Stockings would be nice, and maybe some items to commemorate the discoveries of Shenkuu and Neovia (assuming Neovia becomes a 'mini-world' or whatever after the plot's over - I don't see why not, they've already got the town drawn and there seems to be enough demand for it.
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Ordinarily I'd also like furniture because I like decking the halls of my Neohome during the holidays, but I'd rather they properly release and/or fix some of the furniture that's already been announced in the news. No sense in giving the artists and programmers even more work to have to catch up on. But if they
were caught up, I'd like a holiday candle ("It's pretty and it smells nice, but don't leave this unattended!") or some more tinsel.
I did like the crazy scarf and seasonal Twisted Roses hat they gave out last year. The scarf was perfect for my Mutant Bruce's personality and my Hissi's a big Twisted Roses fan. But I wish they'd make clothes usuable, kinda like toys or instruments. The command could say, "Put this on Druscylla," or something, and your pets could reply, "Yay! I love this color!" or "No way!! I'm not wearing THAT ugly thing!" or whatever. While you can't actually see them wear the item - well, you can't actually see them play with their toys or instuments either, right?
If clothes worked like that, I wouldn't mind getting them from the Advent Calendar and might even buy a few. But since they don't...well, just becomes another pretty thing to toss in the SDB.