I like this game a lot. I think if the shuriken does damage when coming back will be cool. Also, think power-ups like shuriken can pierce through enemies would be a nice feature if you plan on adding after the jam.
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We definitely plan on adding more post-jam! We actually had code written for a powerup that'd increase your maximum health but we never ended up using it! Never considered the piercing powerup though, that's a great idea! Thanks!
As for shuriken damaging enemies during rewind, we actually added that post-game jam already! Sorry that you weren't able to get to try it though!