Some of my favorite solos, dare I say the best, are the ones that have a coherent structure with lots of improvisation and development.
The best way to understand the concept is to study and analyze music. I've found that studying classical music helps you understand what you're playing, no matter what style you play.
Rather than waste time explaining the various compositional forms, I suggest using the terms you know, such as "Theme", "Variation", "Development", "Improvisation", etc. It is not possible to cover all the terms and methods used to create a Composition here, so I will try to explain only some of the possibilities.
* Opening Themes. It can be anything you choose. Develop this melody or groove so that for the audience, this is your Main Theme.
* Contrasting. It can be something completely different from the Main Theme that is played for contrast before playing the Main Theme again. It can even be free improvisation.
* Theme Development. You can return to your Main Theme, or create a new theme based on the original theme. Let's say, play it in a new key. Often the behavior of the audience dictates the style.
* Climax and/or Coda. There may be new material, improvisation, a Main Theme played especially powerfully, etc.
Using dynamics and phrasing is a big plus for you. It all depends on what you want to say in your solo. A good solo always includes elements of contrast. You can start by using a rhythmic/melodic theme, then move into a contrasting section, then improvise, and finally move on to the Coda.
If you don't have time (short solo) to use a lot of symmetry, and thematic development, you can just play the Main Theme, develop it, and return to the Climax.
Remember, this is just the basic outline. There are no set time limits for each section, or the entire solo. You may want to shorten or lengthen the various sections to showcase your own personal interpretations and ideas.
I recommend that you do some research and study some of these popular Classical forms:
* Simple Double Shape
* Ternary Form
* Compound Double Form (Sonata Form)
* Rondo Shape
* Theme with Variations
I think this will give you a good starting point for creating some musical drum solos. Remember, your solo should tell a story, convey a thought, etc.