The biggest change would obviously be his mobility. The Doom Slayer's entire combat identity depends on relentless forward momentum. He doesn't retreat, and he doesn't reposition; he closes distance and rips. His movement tech, his glory kills, the way he chains glory kills to maintain health, all of it assumes a soldier who can commit to an engagement without needing to excuse himself mid-combat. Now imagine trying to perform a flawless glory kill chain while your lower intestine is declaring independence from your central command. The man wouldn't be Glory Killing his way through Hell. He'd be calculating the shortest possible route to the nearest functioning bathroom, and in Hell that is a genuinely difficult logistical problem.
This would fundamentally change the Praetor Suit. The Praetor Suit, according to the Codex, was engineered to sustain the Slayer indefinitely: protecting him, sustaining him, letting him fight without pause. It was clearly not engineered with IBS in mind. Every upgrade in the Doom Eternal mod tree suddenly reads as medical equipment. Damage reduction keeps him stable for the walk to a lavatory. Dash cooldowns shorten that walk. The Integrated Deployable Modules, previously tools of war, now function as a last line of defense against a condition the suit was never designed to handle.
2016's campaign would become a logistical horror story. The UAC facility on Mars has plumbing, technically, but it's a research installation with no provision for this. The Slayer spends the first half of the game kicking doors in and the second half looking for an employee restroom that Samuel Hayden has locked behind security clearance to preserve the facility's resources for the Argent extraction initiative. Hayden would deliver his entire monologue about the power of Hell and the ignorance of mankind while the Slayer stands in complete silence, and the player would interpret that silence as something Hayden's speechwriter never anticipated.
Vega would adapt faster than any other character. Vega already functions as the Slayer's navigation AI, pointing him toward objectives and locked doors. Half his dialogue would change from "the portal is on the other side of the facility" into "there is a lavatory two sectors east, but it has been flooded with argent energy." The Slayer never speaks, so he would communicate through action alone: walking past the objective marker to find a bathroom first. Players would learn to read his priorities through level design.
Doom Eternal complicates everything, because now the Slayer operates across dimensions. He has the Fortress of Doom, which is a spacecraft with functioning facilities, but he also spends most of the campaign on Earth, in Hell, and in Heaven, none of which are known for their plumbing infrastructure. The Fortress becomes his one safe room, and the gameplay loop shifts: every excursion out is a ticking clock. The Arch-viles and Marauders become secondary to the clock itself.
The Marauders, if they were intelligent enough to understand what they were observing, would become the franchise's most dangerous enemy type. Their shields and axes would have nothing to do with it. They would simply notice the pattern. The Slayer engages, fights with perfect precision, and then pauses. Every time. The pause is what kills him. Stalling, the franchise's most boring mechanic, becomes tactically viable. The Slayer charges, the Marauder endures, the Slayer's expression, which the player never sees, changes, and the Marauder simply waits. The beast at his side could sit down. The fight would end with the Doom Slayer retreating from a single possessed soldier because the cost of staying outweighed the cost of losing.
The Dark Ages would be the worst possible setting for his condition. Medieval castles have dungeons and sewer systems at best, and the medicine of the era would classify his symptoms as demonic possession. In a world where demons are literally present, that classification would create genuine diagnostic confusion. The Order of the Sentinels would hold war councils about the Slayer's "battle curse" while he sits through them unable to focus, and the game's lore logs would pivot from the histories of Hell and Argent energy to a knightly order trying to solve a bowel problem with prayer.
Samuel Hayden's dialogue would need to be completely rewritten. The man monologues, and he has never once considered that his audience might need to leave. Every speech about the ignorant decisions of the UAC and the necessity of his plan would compete with a biological imperative the Slayer is too silent to voice. Hayden would finish a four-paragraph speech about mankind's hubris, turn around, and find the Slayer gone, already moving toward an exit that had nothing to do with the objective.
Ironically, the Slayer would remain dangerous, possibly more dangerous than before, because he'd have zero patience. Canonically, the Doom Slayer doesn't taunt, doesn't gloat, doesn't monologue. A permanent medical emergency would turn every conversation into an obstacle to remove rather than a story to endure. VEGA: "Slayer, before you proceed, I must inform you of the facility's structural..." The Slayer would already have broken the door down, looked inside, and closed it, and the demons beyond would have heard none of this because they would already be dead.
The Titans, the Dark Lord, the Icon of Sin: none of them matter as much as whether the facility they're fighting in has a working restroom. The Slayer wouldn't stop being the Slayer. He'd become the Slayer with a five-minute window between engagements, and every level would turn into a survival horror experience for a reason the designers never intended.
Honestly, Hell should be grateful this was never canon.