Use 10 different characters in one day: +10 XP
Use 20 different characters in one day: +20 XP
30: +30XP
…
100: +200XP
It would be a great way to reward people for using lots of different characters and also to reward people who took the time and effort to make their roster larger. Unique characters only, not repeats of the same character. (ie BW5* and BW1* are unique, but if you have two Prof X’s, then those are the same).