I predict Kathy faked her death as part of a life insurance scam, but after years away can't live with the lie and now wants to return. Gavin is going to put the blame for the scam all on her and it's gonna result in a huge court battle with Kathy facing jail for deception. Otherwise it's a case of - after a few shock episodes when the truth of her return sinks in - Kathy being back in the cafe serving bacon butties with zero else to do.
Gavin is described as 'a new evil' but he clearly has no physical strength so his power is gonna be mental. He screwed with Kathy's mind to make her go along with the scam in the first place, and now he's gonna place all the blame on her making even Ian doubt her.
Being Sharon's dad, he'll lure her into siding with him against Kathy and Ian.
I sense it's gonna be big!
(As published on Digital Spy: http://forums.digitalspy.co.uk/showthread.php?t=2095752&highlight=)