I can see why it might appeal to him. By the time he left Everton he'd had two jobs and been successful at 100% of them. A couple of years later he's had four and now has a 50% success rate. After such a sudden fall there's little point in taking over a Southampton, say, once Koeman gets poached and keeping them 6th-10th - what does that prove to anyone?
Dropping down to reignite the Villa, however...well, it'd be special.