As you've got so much to concentrate on with Cessna's production models, here are a couple of the failed prototypes that are mostly unknown.
Cessna 182M cantilever wing prototype
With the 172 evolved into the 177, and the 210 modified to adopt an unstrutted cantilever wing, Cessna tried to do the same to the 182, and the first prototype of the 182M was fitted with a new 210-style wing. Initial testing showed there no significant performance increase to compensate for the additional expense of the new wing and airframe modifications to accept it, so the aircraft was retired after only 4 hours of flight time.
Cessna 187 prototype
With the familiar step up from the 172 to the 182, Cessna tried to mirror this with 187 as a big brother to the 177. The primary upgrade to the fixed gear 177 was the addition of the higher powered engine of the 182. Tailplane issues had always been the Achilles' Heel of the 177, and the new engine only increased those problems. The prototype was modified to have a widened T-tail in an effort to correct the difficulties. But, like the earlier 182M prototype, the added complexity and cost of the wing structure meant the aircraft could not compete financially with the stock 182, and the prototype was scrapped in 1968.