Honestly MP should be the last concern - you need to get enough people interested in playing it first and adding a quality MP experience takes quite a bit of development time away from other things.
With that said, I'd vote for QoL & Polish - character models, movement, and improving the overall look of the world. All of this before working on detail things like grass sway, as how you do that will change based on the look of things. I also think this trumps story for now, as currently those things distract from the experience and would negatively impact anyone's enjoyment of the story - characters hopping around seemingly randomly, those huge ears (don't get those), etc.
Overall I like the concept of the game and hope you can keep pushing it forward!