If Blackbeard really kills Shanks after all these years waiting for Shanks to do something, I'd be really dissapointed. Though I think that is the most likely outcome.
I'm not really sure why it's likely. Outside of one throwaway line from Shanks telling Teach he'd fight him during Marineford (and every other marine), it's never even been teased.
Like, just subscribe to the fact that Luffy and Shanks are destined to meet/fight. It's been specified not only by Luffy (twice!), but also Shanks himself.
Seriously, if you believe Mihawk is Zoro's final battle, then try and place him any other faction outside of Shanks. It's hard no?