Q. Attacker Gets Access to a Router on Internet, Whats Next? ~ n008

I am a regular user here and have been following many articles. I just want to understand, what if an attacker gets access to my router, what possible attacks can be carried out. One I can think of is MITM by diverting traffic... What others options work? Can internal networks be scanned using nmap? or can attacker join the network?

I am really after scenarios here, logical ways to proceed with attacks.