First off, instead of one loop I try to have two main loops/themes/ideas that I alternate between, often called A and B sections. Think of it like the verse and chorus in a pop or rock song - usually different melodies and chord progressions, but they need to feel like one follows logically from the other. I’ll also usually have a “bridge” section - something different that only happens once.

Each time I repeat the A or B section, I try to change it up a bit. One strategy I’m trying lately is to use different synth sounds for each repetition, and then make subtle changes to better fit the new sound. I.e. I might change from short staccato notes to more sustained notes, but following the same general up and down path.

Adding or changing harmonies is another good strategy. For a dramatic shift, change the key from minor to major, or maybe just transpose everything up by a fifth. There are more advanced techniques for reharmonizing that I don’t understand well enough to describe.

One of my favorite tricks is to use the transpose track in Logic to shift everything. I’ll just do the 4 bar loop like you say, keep it going for a few minutes, and noodle some melody over the top, staying in the same key. But then you add some transpose events every 2 or 4 bars and boom, you sound like a genius. You can hear this technique in action on this track I did:

https://www.youtube.com/watch?v=w7bhQVhycAQ

(Around 2 minutes once the bass line kicks in.)

With the approach you have now, just keep adding and taking away the layers gradually. Dynamic change can be as good as melodic/harmonic change.

If I’m doing a track with one chord progression, I’ll change the way its played on a hook section. Maybe one part will have longer sustained chords and another more rhythmic/staccato.

Or I’ll go with more variety and have different chords for verses, choruses bridges and pre choruses.

For melodies specifically, utilising a theme that you then build on can be great. Starting with the main theme and then keeping the same rhythm of the melody with different notes (like the imperial march from star wars does) or utilising the same notes with differences to rhythm, or enhancing with extra notes involved.

Your loop is a journey from point A (the first chord or melody) to point B (the resolution) For variation, try taking different routes to reach point B. Or maybe point B changes to a different chord, something that doesn’t resolve. That will allow you to continue your journey to point C (a new chord) Now you can make your way back to point A, or back to point B, or somewhere else entirely. Enjoy the journey

Removing elements is not quite variation but it is a very good technique. If I have a really busy melody/arp/progression, I’ll cut out some notes and use the reduced versions in the leadup to foreshadow the full element.

Call and response is a classic. If you have 2 or 4 bars you can make it the call (part A) and then add something new for 2/4 bars as the response (part B). You then have the option of keeping A relatively static, and adding variation to part B. Because A will act as an ostinato, grounding the listener back in the motif with each call, you can sometimes make each B drastically different from each other and it will still work.

It also helps to know your theory, so you know why what you wrote works the way it does. From there you can make tweaks - you might have one melody pulling from 7th chords and another iteration of it based on major triads, or you might have your cadence resolve to a V here and a I there, or prevent it from resolving after 4 bars and then repeating it to resolve at 8 bars (doubling the length of the element).

More and more what I’ve learned, is that music production is work - and that slamming your head against the keyboard isn’t a bad thing, it’s what you’re committing yourself to when you sit down to write a new song. But then you get the elements in place and everything feels magical and you forget how rough the start was. So sometimes you might just have to start manually and randomly wiggling notes around, and that is a legitimate part of the process.

One last thing, if you have 8 bars of melody or rhythm and need another 8 that matches it, try to work out the core elements of the original 8 - where does the beat fall, rests, where are the accents, which notes are just filler, etc. Note where upward/downward runs start/stop. Particularly the accents and rests. And then write your second 8 bars listening to the just the rhythm of the accents or something else super barebones like that. You can entirely copy where the accents and things are, or you might selectively change them for effect. But keeping those core elements like accents and rests in place will help the second 8 sound like it belongs with the first 8.

Try improvising before you write new parts of the melodies. There are a lot of cool ways to improvise. You can set up a backing track for yourself by muting all the stuff that limits your melody and playing along to the rest or do what traditional composers do and come up with the whole thing on the piano. Which instruments play in your backing track will affect the way you play the melody too, though I would recommend you don’t do it without either drums or bass in there (edit: because they provide the rhythmic backbone for the rest of the music, whatever is muted you’re not syncing to rhythmically).

I have my own method of improvising: I usually play along to other people’s tracks - the ones I reference, look up to and listen to. Just hook up a synth that sits well in the mix of the song that plays and jam away! You’ll get a lot of fresh ideas and will be able to explore different chord progressions and motions without the hassle of writing them yourself!