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UFC champ Amanda Nunes won't have to look far for her bucket list fight -- she can find her opponent in the comfort of her own home ... 'cause she wants to scrap with her FIANCEE!!

TMZ Sports spoke with the MMA power couple leading up to Nunes' fight with Holly Holm at UFC 239 next week ... and her fiancee, Nina Ansaroff, says she's 100% down to fight her boo.

"We already talked about it," Nina says. "After she gets the belt, I'm gonna get my belt at [115 pounds], and then we're gonna do a super-fight at 125 ... we're so down. We really would try to kill each other."

"Why not? Bring the money home in the same household."

Even being in a lighter weight class, the women know each other's weaknesses, considering they've been training together for years.

As for Nunes ... She's totally on board too, saying "My last fight is gonna be against Nina and then I'm gonna retire."

FYI -- the couple has been together for years and got engaged back in August 2018 ... and Nina says their relationship is rock solid, so there won't be any hard feelings after the fight.

"Afterwards it would be like, 'that was awesome.'"