Here is the often neglected but good and easy method of melodic development called "filling in." The first level of this method is the easiest of all the methods to pull off on the fly. It is THE method par-excellence of melodic development in world music so it's REALLY worth your time to have a look.

The two other methods for jazz improv are below.

The use of guide tones. That's already been covered in my tune "Horizon."

The other and most rare is Holdsworth's use of added tone scales. That's already been covered on this forum too elsewhere.