• I think it'd be convenient to allow the keyboard users to change the keybindings when playing with a second player. It would allow players to play with more comfortable hand positions (especially when the first player wants to use the mouse to aim).
• Maybe allow second player (on the keyboard) have some better general aim. Set a button that can be toggled (or held down like the right-click on the mouse) to lock the player in place, allowing them to aim with the arrow keys instead of moving around. 
  • [EDIT: Alternatively, if we want to retain mobility, we can have another set of keys they can be used to aim AND attack simultaneously along with the keys that are used to move. Makes playing seem a bit more complicated, but I've seen it work before and it gives the player some more options and makes their limitations a tad bit more flexible.] Make it a setting if or in case people want to keep the old controls.

I get that implementing these ideas is probably going to be a pain and take up time when you could implement or fix something else. You don't need to. Most people could probably just use a controller for local play instead. It's just my subjective idea  of convenience, especially since I don't often have access to a controller.  I'm enjoying the game so far, so I wouldn't mind too much if you choose to disregard or notice flaws in my ideas. 

EDIT 2+: Grammar because I'm dumb, sorry!