To expand upon this, I think this is sort of like the reality if Steve got over Banjo & Kazooie somewhat. Again, somewhat.
I think the general Smash fanbase wants Doom Slayer more, but mostly anyone who follows video games knows Skyrim. It is HUGE. And I know Todd Howard is kinda sleazy and Bethesda keeps re-selling Skyrim, but... Todd is right; Bethesda will stop re-selling Skyrim when people stop buying it. Skyrim is incredibly popular and has had a huge impact on the industry.
While DOOM also has had a huge impact on the industry, I think Skyrim has more of an appeal worldwide. Look, I get Smash already has some characters us westerners don't know as well (like Hero), but Smash is made IN JAPAN, so obviously Sakurai probably won't add a character who the Japanese don't really know or like. Takumaru wasn't made a playable character because Sakurai knows he is really niche (and this is for the RETRO character pick nonetheless). DOOM has had a huge impact, but it is also violent. Could they tone it down, yes and I'm confident it could work. But let's look at Steve and Banjo & Kazooie parallels:
Similar to the comparisons made between Steve and Banjo & Kazooie, Minecraft is more popular worldwide and has more international appeal. While Banjo & Kazooie were wanted more in the Smash fanbase, even if you are staunchly against Steve, Minecraft is an unstoppable force that is the most successful video game of all time. There is so much going for Minecraft, that ordinarily Banjo-Kazooie getting squat over Minecraft would've been laughable. But... because Sakurai and Nintendo listened and the fans kept up the support for an old platforming star, Minecraft somehow got passed up. Do you realize how much that says about Sakurai? I know *some people* want to say Sakurai doesn't listen or care, but he literally passed up on having the most successful game of all time getting a character in Smash. Do you realize how much more appeal that would've gotten than Banjo & Kazooie? (Disclaimer: Even though I'm young and wasn't born when the bear and bird were in their hay day, I love them and glad they're in.)
So, looping back around to DOOM and Skyrim, similar to Minecraft and Banjo-Kazooie, there are legitimate arguments for why Sakurai would choose one over the fan favorite (much to people's chagrin). Though, obviously, DOOM has much more of an impact and legacy than Banjo-Kazooie I'd say (or at the very least is actively ongoing, so it isn't that niche). But... hey, like I said, Sakurai does listen so maybe DOOM will be chosen over Skyrim! But also, Doom Slayer hasn't been requested that much (I'd say) until recent times. So, unlike Banjo & Kazooie, he hasn't had long-enduring huge fan support. I'd love Doom Slayer, but a Skyrim character makes a lot of sense.
Oh, and I just remembered that Hero and Geno could also work in this scenario. ...isn't it awesome how a character like Geno is even a possibility? I mean the fact that Geno (an old character who has appeared in one Mario spinoff years ago) seemingly has more legs to be in Smash at the moment than pretty much ANY OTHER Square Enix character should really speak volumes to what Sakurai has made possible. The fact that Geno seems so likely now is just a testament to what Sakurai has facilitated with Ultimate.