Hi @lbm and others here,
Now that there is a "lite" rich content editor (RCE) that was developed for SpeedGrader comments, this seems to me like more of a possibility now. I think having the full RCE in the inbox was likely a challenge because many components of the RCE may not work outside the context of a course (integrations, uploads, images, etc). The Lite RCE provides some much-needed formatting without all of the other features of the full RCE that could be more problematic. With that being said, I'll note here that when the RCE came to SpeedGrader comments, it did mess up the display of all existing comments, which would probably be an even bigger issue with the Inbox. That;s not to say it's impossible to code, but it's also not just a simple addition when all of the different things are factored in (web canvas, canvas mobile apps, email notifications, app notifications, etc).
I do hope some basic formatting eventually comes to inbox though. As @james_whalley said, submitting a new idea specifically referening the Lite RCE may help, but I do also know the ideas process is being revamped right now and a change like thius wouldn't be a quick one, just for full transparency.
-Chris