In regards to melody… Sometimes the tune sounds ok by itself - but obviously will start to sound brain numbing after 1000000times on a loop.
Assuming that you’ve got a bit of movement in your sound - a little bit of chorus, little delay, lfo & maybe a bit of sidechaining: Theres still plenty that you can do.
Try chopping it up. Literally… If its an audio part (or you could always replace a bit of midi in a section with audio…) you could reverse it or timestretch to fcuk it up completely -  or generally destroy it as a bit of a break.
Also, automation is probably goin to help a lot - You can hit the release & make the sound longer or shorter - you could also automate the filter to sweep up.
If there isnt a Humanize feature in your DAW for midi, try going into the midi file & altering the velocity values & note lengths. Better if you do this over 32 or 64 bars. If you change your synth patch to filter based on note velocity - or release on note length etc… you will get a more random, suppose human inputted feel.
Quantise your head off when you want things tight - but sometimes for melodies & other parts, its cool to play in the stuff & record it over 32bars. At least its unique & a little random & takes a while to repeat. Fix the notes after. Try duplicating the notes into chords etc… and adding voices in your synth to make the sound fuller.
FX wise - you could add delay / reverb washes at sctions before breaks & changes in your arrangement to keep things interesting… or even when there is no change at all apart from a new drum element being introduced. Find out about using a Gate to make a stuttering melody out of a Pad Chords. You can use anything to trigger it & it can sound really cool… Like a drumpart or a spoken phrase or a programmed midi part. My Sampler takes a siidechain input, so recently I’ve been messing around with bringing in different patterns into it, triggering the Cutoff or Pitch… Sounds cool (though not always useable…) -  Much better that drawing in Automation.Â
Sometimes its just as handy to duplicate your synth track & make a bit of a variation of the synth patch (save automating LOTS of stuff…) and have that play a couple of notes here & there that you’ve muted on the other part. You could even automate 2 different filter sweeps on the 2 patches, change the glide / release - the possibilities are limitless… just a case of doing it! LOL
Call & response stuff is cool with melodies - you can always drop or higher it an octave in places for a bit of variation - or do it with a pitchbend for interest. You also have the option if it works out -  to double or half the notes (speed it up / slow it down) so that the melody repeats twice in the same length of time it normally takes to play - or twice as long etc… You can even repeat parts of your Midi region or Audio file by chopping it up…
Experiment with moving your notes around on the grid. Moving things left or right a bit can add a whole new dimension to the groove. Sometimes you start listening from 1/2way into the bar etc… & it sounds even better than playing from the start. Its pretty easy to move the melody around to where you caught it. Always happens to me 
Mess around - It mightn’t sound great repeated over & over, but as a variation it might just be considered somewhat perfect! 
