Thank you!! Yeah we had a couple ideas for goals, but we went with an arcade high-score approach so we could focus more on polishing the core gameplay. If we continue working on this, we absolutely want to explore a 3-Act structure with intentionally designed levels.
If you want a challenge to aim for, it is possible to "break" the scoreboard by scoring 9,999,999+ in one round. You really have to master the pattern system (repeatedly perform a sequence of 2-6 steps, such as ABCABCABC) to achieve this, but with enough practice you can score 3 million+ points with a single 47-step combo :P (Although I've only achieved this in the rehearsal mode, my reflexes aren't good enough for doing that in the full game!)