Give us a little green circle for the enemy's ultimate on the TAB menu
It would function exactly like the Summoner Spell icons do for enemies. We don't actually see when those go on cooldown, but we can ping them so that allies know they're on cooldown.
Likewise, simply add something for us to ping to let allies know an enemy already used their ultimate.
This stems from an argument I was having regarding the "necessity" of the chat for player communication, and I realized that this is the only thing we can't ping or see for ourselves. Once this is implemented, any argument for chat being necessary for communication in game will be moot.