URL previews
If you share a URL in most modern proprietary IM clients, instead of the URL is shown a preview of it, with a title, a description and usually a thumbnail too. A possible solution to have that in gajim would be to make gajim fetch the content of URLs and generate a preview from it. While that's probably reasonable for known contacts and private groups for most users, this is a privacy issue in public groups.
The elegant way to solve that is sender-generated previews. Cheogram already does that in a custom way. There is no XEP yet, but it would be nice if gajim parsed and possibly sent these sender-generated URL previews.