For example, you can vary the rhythm or the melody. Does the melody start on the first beat, second beat or rather the fourth beat of the preceding measure? Is the rhythmic accent on the third beat, etc.? ( of course, reaggae makes it ethnic) Interplay between these is endless. Major or minor. Chromatic or movement by fourths. Ascending or desending bass lines.
There are so many factors that become tools to sculp your own musical image.
Jobim used to find all the chord forms that could included a particular note. For example Gmaj7b5 and Faug share a C#. So your c# melody note could span those two chords. and use that pattern throughout a verse. Start the bridge portion of a song on an unexpected chord and use your first chord of the chorus as a Home Target. Now work backwards from your target chord while writing forward from your beginning chord. this will force you to bridge the gap using Chromatics, forths, minors with fifths in the base, etc. until you've created something unique.
Here's one I've just done like that. Song is in A. Bridge starts Dmaj7, Amaj7, Fmaj7, Amj7. Then Dmaj7, Amaj7, Gmaj7, C#7sus/Ab. The C#7sus with the Ab bass takes us back chromatically to A major. And it crosses pop, jazz, dance, reggae, etc.
What I am trying to share is that, in my experience, uniqueness in writing comes from applying the things you have spent hours learning, not something you're going to grab out of the air.
A gymnast with a dazzling routine has also nailed the fundamentals.
Nonetheless, that's a tough assignment given the parameters.