I have a lot of individual calendars connected to my thunderbird calendar (one per collegue plus multiple personal ones). Having them colour coded is great, but I will not learn those 12 different colours by heart any time soon. Hence I would love to have the possibility to add a prefix, suffix, or tag or something, that is displayed inside the entries. Maybe even a little picture (so I can add a collegues avatar, that he uses in out chatting software), but initials or unicode/emojies would help a lot.
I also assume, that having this feature would greatly increase accessibility for coulour blind people, making thunderbird a bit more barrier free.