It's quite good, but if you don't have a second player, it loses a part of its charm.
You could try adding a basic computer opponent (go up and if you notice a rock moving in your direction, move the AI a bit to the left or right).
https://books.google.cz/books?id=V-KEDwAAQBAJ&lpg=PA92&ots=bQ77--nnIY&dq=obstacle%20avoidance%20triple%20raycast&hl=cs&pg=PA90#v=onepage&q&f=false
Sometimes the patterns of the rocks left a wide gap and I could score two three points just by flying up without even turning.
Good idea with the slowly descending barrier, though.
It might be more interesting if some rocks were slower/faster than others. Or add different kinds of rocks.
There's even a potential for powerups (clear a small area around you when you pick it up, get a temporary shield, block some part of the top screen edge, slow down your opponent…)
Speaking of which, some kind of interaction with your opponent would almost certainly make it better. Because so far, you could just as well play a single-player game on separate computers and compare the results.
I would think of ways in which the players could affect each other (such as shooting when you're below your opponent, or changing the rock pattern as it's coming across your half of the screen).