Currently:

• Trello has some stickers.
• Apple now has customizable Memoji stickers, announced at WWDC 2019 Special Event June 3, 2019.

Possible features for this:

• Import stickers from Trello and other apps.
• When importing, replace stickers with Wekan stickers.
• Import Apple Memoji stickers from some app or API.

But I don't know is it possible to use Memoji stickers on other apps like Wekan, because I'm not familiar how they work.

Please someone add links where is more info about related code, sticker images, APIs, anything.