Tomboy is a personality... not a body archetype. Tomboys are girls that go against the stereotypes of what a female is seen as by females themselves.
Also.... Rin's entire character is tomboy. You's character is also tomboy... We've had tomboys for the past 8 years...
Honestly, I hope it's a male so that not only will it prove that Love Live isn't entirely yuri bait and isn't afraid to take risks with an important male character, but also because it might finally settle the dispute between the second year shippers by bringing something new to the table or eradicating the battle with a canon bombshell.