>>46717692Going with Ghost/Grass
Theme would be a haunted forest, player enters clearings in the trees only to see them get blocked by different obstacles themed after gym pokemon. Ex:
One room gets filled with poisonous grass that acts as toxic spikes if you get into a battle
One room is dark with floating lights and eyes
One room has moving trees that open and block paths as you move
When you make it through the forest you see the gym leader, who then battles you with a team similar to the pic.
Winner gets a TM for wil-o-wisp and a badge that looks like Trevanent's face.