What Do You Think About Marcus Returning
Predictable, but welcome.
Mr. Dorns return was not so expected, and is welcome indeed.
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The thing to think about with these "ancient" NPC's, is that in all this time nothing has been able to kill them, and they've been adventuring for years (read MAX level).
So IMO all of the still living supermutants
should be tougher than FO3's Reavers/ and or Albinos (including Marcus); and Marcus should look on the PC practically as a child.
Maybe an escort mission where he helps you out in a tough fight, but to leave his home as a regular follower... Your PC should be able to hold their own against him.