What specific items/pieces would you like to see?

[Pynedor]

There is an official 4-way connector with smooth surface that came in brown and a limited release version in grey.

Yes, the grey one was formerly sold through the PCC. It seems like it has a part number (30 21 8213), but I doubt any of them would be available for ordering now.

[Klickteryx]

Yes, the grey one was formerly sold through the PCC. It seems like it has a part number (30 21 8213), but I doubt any of them would be available for ordering now.

The brown one is, it's 3 euros.
https://www.playmobil.de/ersatzteilsuche?q=30089600

[Klickteryx]

Nothing very rock n roll, but I’d like to see:

• not so much a piece but maybe a Kinetic sand collaboration with pirates digging up treasure

I think with sand that actual sand would be better for digging treasure up. Have a solid sand base as you mention but place it on a sand area or perhaps have another sand base which is a frame with actual sand in the middle. The frame could have a detachable floor so you could fill it up with sand for use indoors and keep the mess down or take the floor out and use the frame at the beach and dig as deep as you like.

Looking at your list of possible steck parts reminds me that the pirate prison came with a couple of nice pieces - door with hole in it for passing things through, a whole wall of bars and a brown coloured cap that covered the top of wall sections.
This thing: https://playmodb.org/cgi-bin/showpart.pl?partnum=30-09-5310

The waves for the ship would actually make a nice regular piece to clip to the bottom of the hull.

[Klickteryx]

Backdrops. The Advent calendars tend to have cardboard scenery, it would be nice in larger sets to have a cardboard sheet the size of the box or even folded in half to be bigger to have a backdrop. This could be a modern town, an old village (with or without figures so it can be multi-time period) or just countryside.

Also a themed play-mat that figures and buildings could be placed on. This could be sold separately and represent a town with roads, paved streets for historic town, or appropriate art for licences like Asterix giving a proper comic book ground for displays to stand on. 

[Nightlioness]

Animal related, of course XD
Long haired cats! I would love to see a more fluffier cat, side head fluff and a long fluff tail. Different colors.
Or cats sets like they did withe the dog breeds, breeds could be: Siamese, Persian, other long haired cat breed like the Norwegian, Russian blue.

[Rasputin]

Mistake