Here is my theory....Meryl somehow rejoins the group. Comes to find that Darryl's character has evolved and has a new "fammily" now. Meryl tries to kill Rick, and Darryl kills Merle with the crossbow.
I can't see what Merles roll is longer term in this show other than to have Darrly kill him.
I agree, Daryl will take out Meryl I'm guessing