I know these are super rare but there have been a few. Ill start with the most obvious one: Holmes vs. Cooney Your turn...
Two Champions involved in a non title bout, Carlos Zarate, WBC Bantamweight Champion vs WBA Bantamweight Champion Alfonso Zamora, on April 23 1977, Zarate was 45-0, 44 KO's, Zamora was 29-0, 29 KO's, it was billed as Battle Of The Z Boys, Zarate won by KO 4.
A couple that come to mind: Cuevas (c) vs. Hearns (challenger): Two acknowledged dynamite punchers in a showdown that people really wanted to see … was Hearns going to really be the rising star that people pegged him as or was he going to get chopped down by Pipino’s hooks from hell? Benitez (c) vs. Leonard (challenger): Pure boxer, two-division champion wunderkind vs. Olympic hero and the guy on his way to being the face of boxing … if he can get past Wilfredo. Patterson (c) vs. Liston (challenger): Sonny had cleaned out the division and at the behest of his manager Cus D’Amato, Floyd had ducked him as Cus tried to protect his charge and freeze Liston out of the title picture. Floyd canned Cus and did the honorable thing by giving Sonny his shot … it’s not looked at as a superfight in retrospect, but this was a huge showdown.
Victor Galindez v Richie Kates, Jorge Ahumada, Yaqui Lopez, Eddie Mustafa Muhammad Roberto Duran v Esteban DeJesus (1974) Antonio Cervantes v Esteban DeJesus, Hector Thompson Jose Napoles v Hedgemon Lewis, Ernie Lopez, Armando Muniz Miguel Canto v Betulio Gonzalez, Ignacio Espinal, Martin Vargas Ruben Olivares v Jesus Pimental, Chucho Castillo