> The really boring approach IMO is to choose a random solid color and put the user's initial(s) on top of it. This makes it pretty unique and much easier to associate someone back to their avatar.

I agree that it's a good simple baseline, but going on a minor off-topic rant, I find it very annoying when this is implemented under the anglocentric assumption of names always being of the format Firstname (Middlename) Surname everywhere, and thus always parsing the initials as "FS". As a result, Spanish people whose naming custom is Firstname(s) 1stSurname 2ndSurname, where the 2nd surname is the "droppable" one, are inappropriately rendered. Director Pedro Almodóvar Caballero becomes "PC", rather than the acceptable versions "PA" or "PAC".