PvZ 2... A third person shooter? Honestly, that largely kills my interest in it. If I wanted another shooter I'd play one of the two million on the market. As it stands, I want a PvZ shooter as much as I want a Street Fighter RPG or a Final Fantasy fighting game.
Take one of the best tower defense games out there and change the genre. All I have to say is... Why?
Because a lot of people will happily switch genres to continue with a unique IP, and it's an easy way to keep things new. If you don't believe me, take a look at the massive variety of "Mario" games that Nintendo has come up with over the years. Huge variety of genres, same IP collection, and people eat them up.