Enemy flight pattern experiment

I've been trying to think how to make shooting and hitting enemies a fun and interesting challenge. I made the enemy ships fly horizontally past the screen to test how it feels, but I'm a bit out of ideas at the moment. It does seem like a minor improvement over the previous way of the enemy ships sort of following the player. I also added a score counter.

The game doesn't feel fun at the moment, at least not past the first 15 seconds of gameplay. I'm wondering which way to go: tweak the core gameplay and try to make it as fun as possible, or add more things such as different types of enemy ships, power-ups etc. I would usually think that the core gameplay must be fun and if it isn't, then it doesn't help adding stuff on top of it. But maybe the game is just too simple to be fun?