Here is my quickly sketched up idea of how a Miz Ambrose feud could work. Maybe not the best one but bare with me.
Miz and Rey continue their feud. During a title defense Ambrose jumps in from the crowd (announcers don't know who he is) attacks Rey and leaves through the crowd. Rey gets a rematch and again Ambrose intervenes. After a while Rey cuts a promo on the situation with the "rowdy fan". Ambrose again jumps the rail and grabs a mic, cuts a promo on Rey, about how he will destroy him, about how someone is paying him to the it and how he will enjoy causing Rey pain. Miz comes out and reveals himself as the one who paid Ambrose, the two beat down Rey.
Rey and Ambrose fight for a week or two, then it all falls into tag teaming, Rey and another popular face (Ryder, Cara, Santino) vs Miz and Ambrose. Ambrose destroys Rey, sending him off with a injury. Miz starts feuding with other wrestlers and over time starts showcasing more face tendencies yet he keeps up some kind of contact or association with Ambrose who keeps beating guys in the mid card/lower card (not squashes, real competitive matches that can showcase him as a psycho). Ambrose also keeps intervening in the occasional Miz match.
Eventually Miz is put into a title feud against someone he is a underdog against (insert Show or whatever heel big man). Miz beats him with the help of Ambrose in a no DQ match. RAW after the match Miz cuts a promo that showcases even more that he is leaning towards going good, he welcomes Ambrose out to thank him for saving him (have show or whoever maybe have been going to break Miz's leg when Ambrose comes out). Ambrose and Miz shakes hands and Ambrose lays Miz out. Savagely beating him and stealing the belt. Leaving Miz to be sent out on a stretcher as he walks of with the IC belt.
It's not perfect and needs fine tuning but it includes why I think could be the basis for such a feud.