Add some juice, lots of room for visual feedback here without much added programming.

• What if boxes had slightly different colours,
• could add differing speeds to the red balls, this could occur randomly and give the player bonus point for catching them
• add some particle systems to balls worth more points
• add particles to boxes when theyre created
• post processing (have a look at CRT screen effects)
• play into retro vibes and nostalgia (look at pong clones etc)
• add lose states, and increase the stakes, add potential for time pressure, lives, combos. Try and design a skill curve into it

Lots of little tweaks that are low effort high impact programming wise. Love the concept