Seeing how I suck ass at anything and everything, I feel that I am obliged to ask other people to do things for me.
This update doesn't seem to need more cosmetics. What it does seem to need are weapons, taunts, and a few more unusual effects.
For instance:
A new pyro primary
A new engineer secondary
A new engineer wrench
A new spy sapper
A new spy primary
A new heavy lunchbox secondary
A new medic secondary
A new medic primary
A new demo primary
A new demo secondary
An all-class melee
A soldier taunt
A scout taunt
A medic taunt
An all-class taunt (group, pair, or single)
Maybe another hat effect
About three weapon effects

Not to mention SKINS! Volvo seems to be obsessed with skins, so a few of those wouldn't hurt.
After that, we already have a basic understanding of what the Mayann story is; which is possibly the search for idols and artifacts that contain australium. Volvo literally just has to put a few weeks worth and a few thousand bucks worth into this update for a few new voice lines, servers. You amazing, artistic, beautiful people have already done all the work for them.

TF2 is dying, as we all know. Or weakening, rather. Overwatch, market prices dropping, and more are evidence enough of this. YouTuber KrunchyNaut has said that TF2 stands on two legs; YouTube and items. That is partially true. The third leg is interesting updates, especially ones featuring entirely community-made items and maps.

I'm sure that an update like "Mayann" or "Iron Gauntlet" could probably spark the interest of new players (which we desperately need, like it or not) and maybe even bring back those who we lost- *COUGH COUGH* Ster *COUGH*.

Anyways, that's a ramble by someone who cares too much about an old video game.

Congrats if you've read this far.