I think some flavor of #Markdown should be integrated into the default webapp of mastodon. It would enrich the users' experience with almost no extra overhead.
That said, I think these should not be allowed:
- adding images `![](URL)` because it would bring things from outside of fediverse which can contain tracking and also can change on the fly without user's knowing
- adding hyperlinks `[](URL)`because of the reasons above and also it helps masking malicious links (e.g `[good.com](bad.com)`