Honestly, the craziest move Marvel could pull is making it a straight-up bootstrap paradox, Doom hid the Ten Rings on Earth himself.
If Doomβs already hopping timelines and playing 4D chess to build his God Emperor armor, he easily could've sent those rings back thousands of years into 616's past as a localized beacon.
That way, no matter what happened to the Multiverse, the rings would ping his citadel the second incursions started hitting, giving him a direct roadmap back to the exact pieces of armor he left on Earth.
nobody noticed that the markings on doctor doom's armor just solved the biggest unanswered mystery in the mcu.
when doom breaks into bruce banner's bunker in endgame encore, glowing runes appear on the floor that match the symbols etched into his suit and the ten rings.
everyone thought marvel dropped the phase 4 artifact plotlines when they moved on from kang. they didn't. doom is coming to earth-616 in avengers doomsday because the pieces of his god emperor armor are scattered across the colliding universes:
shang-chi's ten rings: sending a beacon into deep space since 2021 straight to doom's citadel.
wakanda & talokan vibranium: why shuri and namor are both in doomsday (doom needs pure vibranium so his armor doesn't burn out like tony's nano gauntlet).
the green runes (endgame encore): ancient latverian sorcery etched into his suit to cage cosmic power like hulk.
sentry & the void (thunderbolts*): the dark-matter battery to power the armor once loki's tree falls.
franklin richards (first steps) & the x-men: the reality spark to weld the surviving earths into battleworld.
doom isn't invading earth-616 just to conquer it. 616 has been holding the missing pieces of his suit for thousands of years.
only piece missing is who hid the ten rings on earth in the first place. who's your bet?