Do you know of any au fics where Tony didn’t fall back through
the portal in time and it branched off from there? (Not death fic but a
What If scenario.)

I could only find a few completed ones for this idea, and some aren’t really your ask, but you might like them.  Here you go!

REMEMBER TO COMMENT AND KUDOS YOUR AUTHORS BEFORE IT IS TOO LATE!

The Machine that Kept Him Alive by orphan_account:
AU where Tony does not manage to make it back through the portal during
the battle the Avengers have in New York. This is a one shot of how
Steve lives in the Stark Tower and deals with Tony’s passing. SAD.
Stony, established relationship. Superhusbands.

I’d Rather Hurt (Than Feel Nothing At All) by Maximoff:
What if the portal closed before Tony crossed it? How would Steve react to the loss of yet another significant other?

Don’t Do It by Resacon1990: He wanted to squeeze his eyes shut, to hide from the
image of the closing portal that Tony was still in, whether dead or
alive. A fleeting thought flicker through his head when he wondered if
this was how Peggy felt so long ago, trying desperately to contact Steve
as he flew the ship into the ground at breakneck speed. A feeling of
self-loathing overtook him.What if Steve begged Tony to not go through with taking the nuke?
What if Steve was convinced he was dead? What if it was enough to bring
them together?

The Other Side of the Door by AnonEHouse:
Tony Stark died during the battle for New York, but three months later a
Tony Stark from another dimension falls into Steve’s arms. Tony
desperately needs Steve’s help in order to survive long enough for his
baby to be born. It’s just common decency on Steve’s part. Isn’t it? The
fact that he finds Tony attractive has nothing to do with it. After
all, he’s not the Steve who Tony married. The Avengers are just… well,
you wouldn’t turn away a pregnant man who had no place to go, would
you?

Saga by Teyke:
Tony falls into the closing portal, but he doesn’t make it out the other
side before it snaps shut. Trapped in a universe where time runs oddly
when mortals blink, and accompanied by a Not-Quite-Dead Steve Rogers,
Tony mouths off to a goddess of death, goes cross-dressing with Thor and
Loki, and learns to hate Asgardian engineering. Oh, and he may have
caused the end of the universe as they know it