Continuing an 8-bar theme

After creating a simple 8-bar period or sentence format

What is the next step?

Repeating the same 8 measures again seems to sound monotome

To create some variation I tried to apply the techniques of melodic development, such as inversion, retrograde, ornamentation, augmentation or diminution, and when applying these techniques in the theme of 8 measures, the new material generated was very strange, without logic and without feeling

any tips on how to elaborate the same theme again without it getting weird or too repetitive?

You're doing the right thing here: trusting your gut!

There's no correct answer to your question, though. Take a break and listen to some of your favorite music (or be adventurous and try something new)... I guarantee an idea will come to you eventually. The main thing is not to force anything, and it sounds like you recognize a forced variation when you hear it. That's good news!

Keep at it, and don't be afraid to scrap everything and start afresh. Sometimes I'm amazed at how themes I've trashed have found their way back into my musical tapestry. 😉 

13 hours ago, Tónskáld said:

Take a break and listen to some of your favorite music

I try to listen to different compositions, mainly compositions from games like World of Warcraft, The Elder Scrolls V: Skyrim, The Elder Scrolls Online, The Witcher 3, and other fantasy RPG games;

I have a very good ear, but I can’t identify patterns of shape, structure or anything that helps me with my compositions,
these soundtracks that I mentioned, seem to be very well tied and connected, there is no way of knowing if the composer changed shape or not, he developed melodies or not, I am still confused

Beethoven, to me at least, is the undisputed master in this; taking even the most banal melody or harmonic progression and transforming it in such profound ways.  His variations in C minor and the 2nd movement from his seventh symphony are particularly good examples of this.

6 hours ago, Victor_lod said:

I have a very good ear, but I can’t identify patterns of shape, structure or anything that helps me with my compositions,
these soundtracks that I mentioned, seem to be very well tied and connected, there is no way of knowing if the composer changed shape or not, he developed melodies or not, I am still confused

Sentence and period forms can be expanded to 16 bars, don't forget.

The thing with the soundtracks that you mention is that they don't follow any sort of traditional structure (most of the time) and are often either just short, through-composed tunes that loop, or a dynamic soundtrack where different loops and ideas are brought in according to what happens in the game.

One thing you might consider is that you don't actually need a theme present at all times in the music; some places of just accompaniment and then bringing in the melody or other elements later is useful, but if you're using samples — samples really struggle with that.

It depends on the style you are writing, the form you have in mind...That's right that a 8 bar phrase can be easily extended to a 16 bar phrase (first half-phrase ended in Half-cadence and second half-phrase ended in tonic). 

There are different techniques to extend the musical ideas. One can also start a new phrase with the dominant after the first one with similar motif but from de dominant instead of the tonic, that's an easy way to develop a musical idea. Also possible to start a new phrase from the subdominant instead of the tonic using the same rythmic idea. 

One can also repeat the same phrase with some small variations or change the texture, orchestration or the register (just changing the octave can be an interesting variation) like in Beethoven's Op.13 Pathetic Sonata II mov. The first 8 bars are repeated just changing to the higher octave with some extra filling notes in the accompaniment. 

If you want to go more in the Theme with variations form. Haydn or Mozart have countless exemples of that as 2nd movement in most of the symfonies or sonatas, like in KV331 or the famous Emperor Quartet from Haydn. 

Regarding the you mencionen I never used most of them. I used inversion once in a piano inventioin and augmentation or diminution just a couple of times. I never used retrogradation, I find a very abstract way to develop. I know Bach used it frequently, but I couldn't never find a use on my works...to abstract for me 😅. Ornamentation, of course. I would also add fragmentation, when you use just the begining of the motif, or a part of it. That's a very used technique on the development of Allegro-Sonata form.

If you have a phrase you want to develop, it would be easier to suggest more specific ways.