MySQL：替代UUID作为缩放的PK

An intelligent person told me: if your project becomes successful, you just have to make sure you can solve your load-problems by throwing money at them.

So, if my project goes big, I need to make sure it can scale horizontally, meaning get more servers, application- and DB-wise.

This should work in my case except the problem with primary-keys. I use simple auto-increment PKs on all MySQL-tables. Is there an alternative apart from switching to UUIDs (varchar 36)?

Thanks!