I've been considering adding a "run" button that lets the player move slightly faster but stop firing their weapon during. I may also add dash as an unlockable skill card. Maybe these will be the same thing.
The mark and shield ideas are interesting, I'll play around with those concepts a little. Shields would be another good ability to add as an unlockable skill. Lots of skills like this to add different attacks will also be added soon.
I'm also considering allowing the player to shoot through obstacles, since the main reason I disabled it was so you couldn't exploit the AI getting stuck behind stuff. I might end up doing this only with a certain arrow type though.
One change I've already made which will be in the next update, is that arrows now have a "knock back" effect. This can be really helpful to counter charging enemies by stopping a bit of their momentum.