Instead of oneboxing Giphy, I’d almost be more interested in seeing installation-local support for looping short videos:
- Discourse (plugin) would transcode mp4 / webm versions of gifs and attach them to the post. When viewing the post, only one of the formats—the best one supported by the browser—would actually be rendered.
- Animated gifs from Giphy and elsewhere would automatically be downloaded and attached, the same as ordinary images. Within the bounds of sanity checking, the various formats from upstream would be grabbed instead of transcoding locally and potentially worsening quality.
This is almost certainly a very low priority feature, but It’d still be nice to have. 
This old thread is related, but misses a key point of what I’d like to see. That being ideally these looping videos should have a user experience that is exactly identical to directly attaching a .gif, aside from automagically consuming less bandwidth (and/or being higher quality).
Showing video player controls and invoking “video” presentation semantics is… very 
.