Question : What is the best schema & DB role for a user (app) that I only want to run a SP that creates another user?

What is the best schemas & database role for a user (app) that I only want to run a stored procedure that creates another user?

Would it be different than one that does not create a user?

Answer : What is the best schema & DB role for a user (app) that I only want to run a SP that creates another user?

I wouldn't suggest making its own db_owner user. You could create a user and map their role directly to the stored proceedure and the database (or all fields) that the stored proceedure accesses, i.e. db_datareader/db_datawriter (if thats all the proceedure does).