I strongly feel that Doom Slayer could well be Fighter #5 of the current pass. The reasons for why I believe so include:
1. The fact that DOOM pioneered the FPS genre, and the current Fighters Pass in a way has included characters hailing from series which pioneered genres, or features within genres (with exception of Joker perhaps).
2. DOOM games on Nintendo hardware throughout the years, with DOOM Eternal, and DOOM 64 coming soon.
3. The DOOM Eternal interview of Marty and Hugo at this year's QuakeCon, where when asked if they would bring the Doom Slayer to other franchises such as Mortal Kombat, soon afterwards the idea of perhaps seeing him added to Smash was mentioned.
4. An interview to Pete Hines a while back in which when asked if they had talked with Nintendo about Smash, he gave a brief reply ("Oh, yeah"). This may not mean much but the answer seemed too sure.
5. Pete Hines appearing in the 9.4.2019 Nintendo Direct announcing DOOM 64 (I know, he was just announcing the game coming to the Switch, BUT, the fact that HE was in it is pretty... suspicious (imo).
6. He would be the "radical" addition to the roster (like when Bayonetta was added in Sm4sh, only in this case, it would be bringing a more brutal tone... without the gore of course... cause Nintendo).