Doom Slayer’s issue is his lack of relevance in Japan. Apparently Japan is pretty opposed to hyper violent games, so legendary franchises like Doom and Mortal Kombat just lack any real significance there. In general, there is a huge lack of western appreciation in Smash given the cultural differences. The only western characters there are tied to DK, Minecraft and Banjo. The sad part is, Sakurai does acknowledge the significance of Doom in some interviews, but I am guessing the overseas barrier between Nintendo and rights holders caused him to be left out.
As for Fire Emblem, I don’t believe it is fully Sakurai. Sakurai’s main picks were Marth and Roy in Melee, mainly Marth, simply because he wanted FE to have more of a presence. Roy was just an easy clone to represent the newest game. From there it was the Pokémon cycle of represent newest title with Ike, Robin, and Corrin. Sakurai didn’t even want Corrin in, but the Smash team thought they could design him to be interesting, plus I am sure Nintendo’s marketing department wanted it. Lucina was added as a costume, but they decided to go full character given extra time and her minimal development. Roy was only added back because Melee fags hammered on about it. Then Chrom was added because people bitched about leaks being false and Chrom not the character in Smash Wii U. Finally, we have Byleth, which again, may have just been a Nintendo marketing decision, especially given the fact that Nintendo apparently choose the DLC. FE is just in that weird position like Pokémon where each entry has a new cast, so Smash places a character of the most recent in every entry, and just like Pokémon, when they stop cycling and start bringing back characters, it leads to a mass army of characters that only exist because of conditions at the correct time.