Following ShowT

RenderTreeT.scala

@typeclass trait RenderTreeT[T[_[_]]] {
  def render[F[_]](tf: T[F])(implicit del: Delay[RenderTree, F]): RenderedTree

  def renderTree[F[_]](delay: Delay[RenderTree, F]): RenderTree[T[F]] =
    RenderTree.make(render(_)(delay))
}
object RenderTreeT {
  implicit val fix: RenderTreeT[Fix] = new RenderTreeT[Fix] {
    def render[F[_]](tf: Fix[F])(implicit del: Delay[RenderTree, F]): RenderedTree =
      del(renderTree[F](del)).render(tf.unFix)
  }
}

This would replace RenderTree.recursive, in a way that allows it to be used to constrain T as in def foo[T[_[_]]: RenderTreeT].