D&D 5E PVP arena

[Gardens & Goblins]

To be honest, you're already tailoring the battlefield to the casters. Wide open area for those ranged spells? No roof for the druid? Yup!

So aye, don't worry about throwing the monk a solid bone. I mean, the entire thing could take place in a 70x70 area. That would make things very interesting! Fingers on those fireballs, backs to the wall!

 

[77IM]

You could also hunger-games it up, like: at the start of round 3, an angry fire elemental is released from each corner; at the start of round 5, a torrential flood is released from the two far ends (extinguishing the elementals); at the start of round 7, every other 10-foot-square becomes a 50-foot-deep pit (with a grate at the bottom to drain off the flood water); etc. This way if the fight becomes a static slugfest of ranged spells, the contestants are prompted to move a bit.