From a game design standpoint I see why they would make that call. The establishing of the villain-mayhem archetype in standard falling into grixis colors here feels like a flavor loss.
Sultai as we know it in Tarkir isn’t an apt depiction of what doom should be. There it feels like an extension of established Golgari territory; the connection between life, death, and what lies beyond. Doctor Doom should be Sultai representing his desire to build Latvaria by advancing medicine, magic, and technology, to be the dominant power. Growth/progress, intellect, and dominion.
I’m not a fan of Universes Beyond- I’m especially not a fan of universes beyond when it mangles the DNA in a way that runs counter to the established feel of the game- (TMNT, Marvel). However if they are going to be bringing these characters into the game I’d far prefer it if they’d be forced to fit into the established world of Magic the Gathering rather than changing Magic the Gathering to fit the characters.