Absolutely, I've played around with this. There are tons of open source libraries to determine gender from name. Obviously it cannot be 100% correct, but it can provide a guess
I think it's interesting how much thought goes into default avatars, and that it can be used as a marketing tactic. Thanks for providing link to Twitter as well. Guess I'll have to think more about it in the future.
Honestly? Nothing really. Well, you don't have to think about the code part but besides that? Nothing. Actually it would be more performant to use CSS and HTML (or SVG?) to generate avatars. I have a core package (used to generate the initials), and if used that with html, it would be far better.
I use the service for our internal products, as gravatar fallbacks