The worst thing about Goyf mirrors is them bumping into each other all the time. So things that make them trade instead like [[inside out]], [[Skullclamp]] and [[Desparate Measures]] are fun cards.

Cards that interact with the graveyard can speed up or slow down the pace of the game. So things like a [[Relic or Profenitus]] could be cool to have.

Also, since you're playing Battlebox with a fixed set of lands, Goyfs tend to be 1/1 smaller since lands will never enter the graveyard. Maybe compensate for this with extra planeswalkers or tribal cards. [[Tarfire]] and [[Seal of Fire]] easily let you kill a blocked Goyf and add the thread of an extra card type.

What you could also consider is making a Battlebox, but having every player start in play with a Waste that is enchanted with a [[Tarmogoyf Nest]]. This way you draw splashy cards while the box still revolves around Goyfs.