I loved the concept of a bunch of outcast samurais fighting for justice. Loved the costumes, they were bad ass. But it seemed like that movie was just a big series of missed opportunities. Here are the things I would have changed:
Less standing around and talking, more 1 on 1 fight scenes (I think there was maybe 2)
The guy in the silver looked cool but didn't do much, he would have been a great main villain
The witch didn't do a lot either, she could have been a lot more destructive and a lot crazier... maybe a wife for the guy in the silver
The sword that Snoke (or Snoke's twin brother at least) gave Keanu Reaves could have had a great back story, like Ryu's dragon sword in the original Ninja Gaiden.
Keanu is a great actor, but wasn't really a good fit for that part. They should have had someone that was good with weapons be the main character.
Other than that, not a bad movie, just could have done so much more with it.