Feingold was itching to run for president in 2008, and could be thinking of making a run. But if he was thinking of 2016, wouldn’t he want to have a gubernatorial victory or two under his belt first? Maybe he’s thinking of challenging Obama next year—he could run to Obama’s left while still being viewed by moderate Dems as someone “electable.”
No ... governing is messy, especially with a GOP legislature, and there's a good chance he'd be proven either impotent or be forced to compromise his radical socialist beliefs. Running would also be chancy - if he lost to Walker, he'd be done.
Democrats don't really care if their nominee has governing experience. Their primary electorate is really looney tunes now.