I understand that you want to have either animated stickers or animated emojis.
For file uploads, Discourse supports webp by default, including animated ones.
As for emojis, it supports animated gif and png, but not webp (either animated or not). Plus, the emoji would have the same size as the other emojis.
Example: 
Supporting webp format for emojis could be a feature request. But they won’t be stickers (mostly talking about the dimensions).
edit:
it appears that the format limitation is only (or partially) set client-side when uploading an emoji.
Changing accept=".png,.gif" for accept=".png,.gif,.webp" works, and saves a perfectly valid webp emoji (animated or not).
I’ll open a feature request about this. 