You both are correct in some ways, however, you could simply lock this down as well. (Include Secondary Fire into the options for 16 total ways to screw with your* server)
One being any M2 action. Overweight sandwich and Rifleman zoom are pretty much unrestricted. Firebug airblast, Demoman detonation, Constructor autogun drop/destruction, Healer needle fire, and Q/C knife are all classified as Secondary Fire. Hence some unbalance in this, however the primaries are more of a concern at all. (except Demomans detonations are a third of the ordeal) Meanwhile Rifleman is moves up in threat depending on the situation, and Firebug loses all possibilities to flare
Rumrusher is right able the 'wind-up' though as I spotted this out instantly when I tried out his FireTaunt for a bit before I moved to FireJumpTaunt.
Also, if all options are bound to a key, the player then has to time their action to right before the elevation they desire. It's not easy by any means, and moving levels via 'stairs' is actually slightly insane. Majority of jumps I've been able to complete, so there should be a worry unless it's some advanced map. Besides, if this actually becomes a server-sent plugin, (can't do this effectively with a client plugin if server host) the host would have to plan out their maps as per usual just with more discretion.
What would be greatest thing about this plugin, is if you can modify the options on the fly and not have to restart the server. AKA the host has it normal for a while but then while full for a match in arena, they change right before the next round to FireTaunt and a system message is displayed as "Special round! Both Fire and Taunt are bound as one! Beware!"
This is kinda advanced though so I doubt it quite highly of what we could push through GM past presets before server boots.