Betty had her chance to confront Don/Dick head-on but adopted a more calculating plan B when she decided to go to sleep. 

I think the season will end when in the midst of some seemingly benign conversation at the dinner table, after Don says something and Betty responds by saying something like "Can you explain what you mean Dick?"  Not enough sizzle?

Yes, maybe some question like that--"Who is Dick Whitman?--as they watch the JFK funeral on TV and as Miss Farrell rings the doorbell.

what a tidy package of intrigue that would be!

What about the Army buddy running into Don and Betty or Don and his colleagues? I'm waiting for this character to pop up in some completely unexpected way.

What if Miss Farrell's brother gets to know the  Army buddy (as a neighbor, co-worker or landlord) and BOTH of them run into Don and Betty? Army buddy exclaims "Dick Whitman" and the brother says "Don Draper" simultaneously.

Ha ha!  You're all wrong!

There's already been a cliffhanger involving Roger, so that seems unlikely.

Bert could die but who would care?  Same with any of the characters.  It has to be Don or Betty.

It's not going to involve Don's past being exposed, because that's already been done.  Betty knows, Roger knows, doesn't Bert know?  It could be something involving Hilton.