It's not possible to improve cruise speed/range/economy without compromising short field performance - short of finding a power plant which gives more power without weighing more, and a diesel most certainly won't deliver that.
Nobody "big" is going to stake their N American business on a Thielert engine, currently.
I would argue that Cessna's US sales are there largely of their built-in place within the "American Dream"; otherwise one could be charitable and say they sell because they offer good short field performance and are rugged, are very common, and a lot of people like high-wing. I don't think this package can be really transformed; there is no engine technology that would do that.
An incremental improvement is all you are going to get.