"I want to fight the heavyweight champ of the world," Iran says ... "I'm looking for that."

If you're unfamiliar ... Iran was actually a world champion boxer back in the day -- with 2 victories over Tommy Hearns and a near win over Roberto Duran on his 43-19-1 resume.

But, even though the guy hasn't fought since 1999 ... he says he ain't ready to hang up his gloves yet -- 'cause he wants one last fight with Wilder!!

Oh, and he says he'll win that, too, telling Wilder, "I bet I knock you out!!!"

By the way, Iran is Saquon's great-uncle, and Saquon's credited him in the past as a big reason why he's become one of the NFL's toughest running backs.

Which only means one thing ... better watch out for Saquon's high-school-aged little bro in a couple of years!!!