Trying to finish this off now / en route to GDC. It's a game Ashley Gwinnell and I made at a gamejam in London earlier this year.
It's a shooter. Both players autofire right. They kill waves of enemies which drop pickups, worth big points.
But tiles are constantly teleporting in and out which affect the bullets: mirrors reflect them, and T-junctions duplicate more..
Players can be killed by their own shots (and each-others), but only once they've hit a tile (i.e. they can't shoot each other directly).
The idea is that you get ever-shifting grid networks of shots, which players must get through to collect pickups.
Here's an early screenshot (it's actually much further on than this - just making a thread now so I'm ready to submit later!)