Yoshi-P has said that he was playing other games to get inspiration for this new MMO. How about one of the most classic Western RPGs yet. (Ultima 3)

Make the grass a monster.

No, I'm serious, It's already ugly, make it a monster that must be felled.

Course, the idea of the difficulty behind such a creature as grass (or its cousin, Floors) is that being everywhere at once, it is camouflaged in its natural environment (lawns and fields) players will be challenged on where to hit it, their only clues being the direction of its attacks.

Of course, not all grass is this malicious, but that where the psyche out is... where is the grass hiding, it could be anywhere....It could be everywhere....

I imagine such a trick could only work if there were invisible monsters that only became selectable under certain conditions,

But we can work out the mechanics later! bring in the most pervasive fiend in the new realm.

BRING IN THE GRASSSSS!!!