[gus_massa]

Looks like an interesting idea, but I have no cue about music to help you.

Anyway, anecdote time: My wife used to play random chords in the guitar. She explained me that if you follow some rules it would sound good. I don't remember the details, something like

X -> Y or Z

Y -> X or W

Z -> X

W -> X or Y

where X, Y, Z, W are actually A, B, C, D, E, F or G, but I can't remember which ones. (Did I mention I don't play the guitar?)

So I made a program that played in the speaker 4 notes per chord, and followed those rules to select the next chord. It was surprisingly good for the low effort I made. It was not fantastic or groundbreaking, but not horrible at least.

[tgv]

Music theory covers that. Simple harmonic progressions just work. C, F, G (or I, IV, V) is your basic starter progression. You can insert some other chords, and substitute chords (IV -> ii, V -> iii), etc. It'll sound good, but also predictable.