I like the idea of projectiles wich can kil you, the high number of ghost makes the game unachivable but not frustrating, wich is nice. Perhaps having upgrades on your weapon could break monotony.
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Increasing speed/damages, having some effect such as a piercing attack, slow, explosion or a double-side attack (this effect could aslo return against you considering the fact that you can kill yourself). Enemies could also be better with time (back shild, more life, spawning other enemies when they die).