The smart move would be to franchise him, because lets be realistic he's not going to sign something short term and without a lot of guaranteed money, especially given that half the league has money to burn and decent QB's are few and far between.
Right now the Redskins have all the leverage. If you franchise him and he duplicates his season then he has all the leverage, and players do NOT like getting franchised so you've sort of burned a bridge unnnecessarily.
However, if you sign him to something like 4 years/40 million with incentives that push that to $60 million or so, he'd be dumb not to take that right now. Worst case scenario, you've spent $10 million a year on an average QB which is just about in line with the rest of the league and he would not be a terribly hard asset to move (or contract to dump) if you need to.
Can you find me a QB in the league who has proven as little as Cousins? The dude had one good year, pumping his stats against bad teams. He doesn't need $100 million.
Nobody's talking about 100 million, but Cousins passes the eye test with flying colors. 4th year breakout for a QB is just about right, especially when you consider he only played a total of 14 games his first 3 seasons. 14 games of live experience is not enough to sufficiently judge a young QB. A full season in his fourth year is far more likely to be an accurate indicator of what he is.
I don't know, I just see Drew Brees redux here. Again it's more about the eye test than anything else.