One thing you can do to make a simple harmonic progression more interesting is by altering the fifth of the triad chord.
Most people think adding upper extensions like the 7th, 9th, 11th, and 13th, are the only way to make a chord harmony more interesting, but altering the inner core of the triadic chord itself is a great way.
Take a look at the progression below:
Notice how the simple I-ii-V-I progression was repeated again but with a b5 of a #5 (or both!).
Some alterations way not work, so an in-depth discussion on what is the correct way to alter certain types of chords will be done in the future.
Till then, experiment!
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