One of the dimensions Mark is stuck in is a parody of Spider-man and Doc Ock.
One of the dimensions Mark portals into seems to be the DC universe where he is seen sitting on a rooftop talking to Batman. Batman's cape is visible and Mark comments on his pseudonym being a bit "lazy."
Mark, his mother and father all have black right eyes.
One of the universes Mark is transported to appears to be the Walking Dead universe, also created by Robert Kirkman who created Invincible.