Thinking more about it, I don't find Doom Guy and Master Chief redundant at all. Completely different franchises. Similar character designs, sure, but it's not that bad. Doom invented the modern FPS genre, while Halo presumably used Doom as inspiration while building something entirely new that revitalized and further popularized the genre. While I personally don't want them because first-person shooters aren't my thing, both characters would be excellent additions to Smash. Plus, our only shooter reps are the Samuses, the spacies, Mii Gunner, Duck Hunt, Mega Man, and Inkling; with Duck Hunt being the only first-person one - a game very far-removed from what the genre would become. We could really do with the two most iconic FPS characters in Smash.
That said, I figure for most people, it's not that they find the two to be redundant, but rather, Nintendo might. Adding two western FPS reps who have similar designs does sound quite un-Nintendo (Joker, Hero, and Byleth all have drastically different designs, after all), but they are also the kings of left field, so who knows?
To keep things on-topic, man, Geno is also a shooty boi. And now I want a Mario FPS. dangit! Would probably be the only FPS I'd be interested in playing