If they intend to do a Mortis arc-based story for Ahsoka, they will need to get the audience that did not watch The Clone Wars up to speed on it. Somethings can be waved over, but the whole arc was kind of lore overwhelming. A flashback might be in order, but that would be animated....unless.... There were only three major characters involved in that story that were not the Mortis 'gods': Ahsoka, Anakin, and Obi-wan. If you want to make a live-action flashback or a "last time, on The Clone Wars" thing, you have the needed actors in Greenblatt, Christensen, and McGregor. Of course the Mortis Arc originally also gave cameos for Shmii Skywalker and Qui-Gon Ginn, but that is not strictly needed to tell us what is happening.
As for Padme. She was a good friend and mentor for Ahsoka on how things are outside the Jedi Order. At least in terms of politics, friendships, and that not everything is black and white in the galaxy. How would Padme come into the story is another question entirely. A cameo within Mortis like those of Shmii or Qui-Gon is possible, though what lesson or path would the light or dark side try to sway Ahsoka via Padme is unclear. If Anakin can manifest in Mortis like he did in the World Between Worlds, could he bring Padme to manifest? Or would the Dark Side temp Anakin via Padme? If they all enter Mortis, what does Sabine see? These people mean nothing to her. She never met Padme or Obi-wan, and only met Anakin as Darth Vader. Many questions and many things to get across to the audience if they go to Mortis.
