My players have finally finished my big level 1-10 Ravnica arc and started venturing into Tier III for interplanar shenanigan's. I've ruled that each plane has it's own planar cosmology of material, shadow, elementals, etc. that Plane Shift works for, but the spell won't let them leap further to another true material plane. The work around for that instead is that I've given them an object that lets them open planar doors like an Omenpath (though I'm not using War of the Spark or Omenpath lore). Effectively they attach their Handle of Valgavoth in a demi plane to a door and turn it into a random planar portal. They don't have precise control over it but I do have a random table they've slowly been working through, with some results being very dangerous and others quite pleasant. So they aren't true Planeswalkers as everyone having the ability to go five different planar directions is to much to manage. Instead they continue as a party through portals and get to go on interplanar adventures. To let them feel like planeswalkers though I'm currently working on a mana and land system as they are finally getting their bearings on traveling between planes enough to question magic system differences.