Usergroup tagging - the ability to tag usergroups with @usergroup_name notation. Very useful if you want to tag a specific group of persons, so you don't need to type each name manually.
While there are already - very - few addons on it, I would like to see it adopted as default in XF, as these addons usually not scale very well with multiple tagged usergroups, and often full of minor issues like sometimes the notifications not being always received. I was also talking with a dev of one of those mods telling me he was limited because XF tagging system didn't really work well with the idea of tagging usergroups (archaic behavior sometimes afaik, scalability issues, etc.). Maybe was it proposed before though, in this case my apologies, didn't find much about it, surprisingly.
While I doubt scalability could be much improved (stays a query to be performed), I think default support for it would permit to have a more solid implementation as time goes by.