Trees, like the tunnels under bridges at Central Park.
What's funny is originally there was going to be a hub-facing burger restaurant nearly butted up right to that wall, so there wouldn't be much room for too many trees back there anyway... but there will be lots of trees on either side of the portal regardless to block our view behind it.
Also, similar to what we're seeing with Monsters' portal right now, there's more theming coming to this portal, especially its back side, to help fill in that inside area.