Top tier appears to be fifty gifts sent. That's not all that high, and I don't know if it includes gifts that were sent in previous years (this is my first winter holiday here).
So I suggest 100/250/500 tiers.
If we have a new partner every day for, say, twenty days, five gifts to each partner, that's already 100. Plus it's not like there's an item/title reward, just the achievement itself :)
The problem with this is that people spam with junk items to get the achievements, so I'm not sure it would be a great idea to upgrade the number...
Oh god no. Isn't there enough drama surrounding the MM already? I really can't afford another 450 decent presents, so I would buy cheap stuff. Followed my my MM victim on the boards: mummiiiee someone gave me 450 cheap things i wanted expensive things!

I asked for this last winter, and was ignored by staff...
http://subeta.net/forums.php/read/1677/Mysterious-Melody-Achievement-Tiers
We don't want to encourage spamming cheap / useless items to your MM, this achievement is unlikely to get more tiers :).
💖 ✨ 🤗
^ You have to admit he has a point..same with sending wrapped gifts sometimes. Sad, but true.
If there's so many complaints and so many people disappointed why do we even have Mysterious Melody still? Or more importantly, why do people keep signing up if they're constantly angry or disappointed about it?
I'm bummed that there won't be any more tiers added. Even on my worst days I still think it's really fun.