Why not just reusing the Custom Emoji NIP for this? I don't see an issue and we already have clients that implement both sides of it.
This is a really rough draft, but I think something like this might be a good start to allow users to collection reaction gifs and memes and have them synced across clients
https://github.com/hzrd149/nips/blob/image-collections/652.md
CC nostr:npub1hee433872q2gen90cqh2ypwcq9z7y5ugn23etrd2l2rrwpruss8qwmrsv6
Discussion
I don't want gif packs showing up in my reaction emoji picker. gifs are generally bigger images with text or memes. and custom reactions are smaller images for "liking" a note
Also NIP-30 does not have support for thumbnails which is going to be very important for larger gifs. IMO a new kind with "imeta" tags is going to be much easier to work with
