Depth is an issue for sure, but I don't think just slapping extra weapons would do much to fix it.
Think about it: what would be the point of shurikens? Any damage they deal would be not enough to be useful, you already kill enemies in 2-3 hits (or if shurikens dealt meaningful damage they'd be overpowered as you'd have a ranged weapon to avoid most danger).
I tried making a "level 2" of sorts with this vertical slice, so in terms of weapons, abilities etc. it's very bare, just the actual core of the game. But well, the core is flawed and just feels "off" to me.