It was going to be in the same vein as their Sonic and Mega Man comics.
https://twitter.com/IanFlynnBKC/status/1166822538242117633
Sypnosis for the pitch:
- The springboard was for a 4-part mini that would hopefully turn into an on-going. It had the same approach as Archie!Sonic & Archie!MegaMan at the time: narrative-driven adaptations of the games with stories bridging the canon. It would adapt the original Mario Bros. game
- It started off with Mario & Luigi being summoned to Peach's castle for a plumbing job. Luigi was excited for the career-defining contract, Mario less enthused - a job's a job & he's bored of their career. They arrive just after Bowser has ransacked the Mushroom Kingdom.
- With literally no one left to save the princess, Mario jumps at the call to adventure. Luigi less so. They stock up on what power-ups are left after the invasion and embark on a journey through Worlds 1-8, burning through their provisions since they're novice heroes
- They reach Bowser's Castle in W8-4 and Luigi calls it quits. This is suicide. They're PLUMBERS, not heroes. Mario refuses to give up - too many are counting on them. They part ways, Mario storming the castle.
- Mario makes his way past the traps and firebars to his first showdown with Bowser. The King of Koopas wants to know who's been causing so much trouble for his troops and it's...this pudgy guy? Oh well. Anyone who challenges him meets the same fate.
- It's a knock-down, drag-out fight on the bridge over a magma moat. Mario is too nimble for Bowser to land a solid hit, but Bowser is too tough for Mario to actually hurt. They're at a stalemate when a scream is heard, building from deep in the hall...
- It's Luigi, screaming in blind terror, running thru the castle as fast as he can. Over the bridge. Past the baffled combatants. Into a wall. Which knocks loose the decorative axe. Which cuts the bridge.
- Mario bounces off the falling Bowser to safety. Mario and Luigi reconcile - yeah, Mario may have gotten in over his head, but Luigi could never abandon his brother. Together they rescue Peach who shares a bashful love-at-first-sight moment with Mario.
- The trio returns to the Mushroom Kingdom and begin to rebuild. Peach names Mario & Luigi royal plumbers. Luigi is overjoyed - they're set for life now! No more adventures! Cut to the stinger ending of Bowser crawling out of the magma moat.
According to his last tweet, he heard through the grapevine that Nintendo of America loved it, but the main HQ at Nintendo of Japan were nowhere near as keen on it, so the pitch died there. I'm guessing Miyamoto was the one who personally brought down the axe.