If Delta Chat could use Jirafeau for large file transfers or some other decentralized file-sharing server type, then a new feature for video messaging could become possible. Jirafeau supports file encryption and file self-destruction after the first download, so that might work for decent security.
Another idea would be to do it similarly to how the audio messages currently work, but with video and also a cut-off time to keep the video messages below a certain file size that is supported by the email server. Some may be 5Mb, some may be 50Mb. That might be a setting, too.
Skype used to have a video messaging feature where users could record a video message and send it to the other person. The way it worked was that the video would be saved to the sender’s Microsoft OneDrive cloud storage and the link would be shared with the other person except the Skype client on the other end would format the received link as more of a “play video message” button as it recognized what the OneDrive link was. Video calling over Jitsi is great of course, but sometimes the other person is not available and you may still want to communicate something audibly & visually without bothering to affect the person’s availability.
Cal Newport writes about how E-mail Is Making Us Miserable partly because of people getting too much of it, but… “We’re also, it turns out, really bad at communicating clearly through a purely written medium—all kinds of nuances are lost, especially sarcasm, which leads to frustrating misunderstandings and confused exchanges.” Maybe video messages would help that.