I'm a hobbyist composer and I just started reading Piston's Harmony handbook to recover the basics I never learned. One of the limits I'm trying to overcome is my inability to conceive melodies that can be arranged in a "theme and variations" piece: many melodies I compose are very regular and 'closed' and it's hard for me to imagine them reproposed in different ways (like they would be e.g. in a fugue). I want to stress that I tend to absorb much from the music I listen and I passed many years listening mostly pop / rock / metal.
In the chapter about Function and structure of melody I discovered the difference between a tune ("tunes have a 'closed' internal structure: after their last phrase they don't imply any prosecution") and a symphonic melody ("... is to be used in a more ample work [...] usually doesn't contain strong cadences on the tonic; the absence of a conclusive end allows the symphonic melody to go on, when necessary").
I think my melodies fit the description of the tune, and I seem to recognize in the symphonic melody the features that I desire to exploit. Is my intuition right? Is there a way to turn a tune in a symphonic melody? Am I missing something obvious in my inexperience?
[PS] Sorry for any mismatch between my "quotes" and the original Piston text: I translated on the fly from my mother tongue, the terms may not be exact but I think the core ideas are clear.