The ability to download mods directly to a player joining the server, much like with the server's resource pack. It's a feature that has seen use in a variety of other games, and yes there is always a risk for such a feature, but I believe that it wouldn't be a tremendous amount of work to implement for private servers wishing to make things easier for the less tech savvy players. I'm aware about being not able to mount them as you join the server, all that is usually done at start-up I believe.
Is it even possible with any of the modded server APIs? Or is it something completely inaccessible for fear of malicious use to corrupt a user's .minecraft directory?
By the by, the main reason I thought this would be cool, is because I was testing out a fabric server with items such as Vanilla Hammers and Adapaxels, and they performed their tasks fine even when viewed from a player in Vanilla( couldn't see the tools but the fact was there ).