After hearing of the rumors going around saying RDR3 started development I would like to share a theory of mine, (don't take anything I say as fact this is all speculation and my opinion)
Mac Callender is a character briefly mentioned throughout the game during discussions and cut scenes with other gang members, during chapter 1 he was mostly grouped up with Sean as they both ran off by the time the Van Der Linde gang saw that they couldn't break through the Pinkerton army and retreated. since then he isn't spoken about too much until Chapter 2 in the mission "A fisher of men" where whilst fishing with Jack Agent Milton and Agent Ross interrupt the lovely scene to try and bribe Arthur obviously Arthur declines but in this scene, we get confirmation that Mac was "shot up" by the time Milton got to him saying shooting him was more of a mercy killing. which aggravated Arthur showing they had a sort of connection.
And that's where most people end the Callender Boys story but I think their story runs much deeper due to the fact they seem to be early members of the gang dating back around the late 1870s to early 1880s due to the fact they joined the gang 2 - 3 years after it was founded by Arthur, Dutch, and Hosea. this shows that if we were to get a game about Mac Callender it would show the early times of the gang and how Dutch and Colm turned their friendship into a bit of a rivalry.
Adding to the game aspect of things if you listen around camp you'll hear stories about the Callender boys such as one from Uncle where you hear about how Mac single-handedly beat 15 sailors in a fistfight and to be honest to me that sounds like a fun mission another story you can hear about them is when the Van Der Linde gang decided to make camp in town up in Chicago someone stole Hosea's Saddle which enraged Mac causing him to threaten to slaughter the entire town if the man who stole it didn't own up.
The Connection between the Callender Boys and other gang members of the Gang is quite strange due to them having diverse personalities but through some camp interactions you can learn a little more about these fellows during the campfire you'll hear Lenny showing a strong dislike for Davy with no real opinion on Mac opening up to a High and Low Honor system. whilst Bill Williamson thinks otherwise having high respect for both of the brothers Charles Smith seems to have a little bit of dislike for both of the Callender boys since Charles joined the gang a bit early not getting to witness their feelings only seeing their bloodshed then again Micha Bell also a member who hasn't lived with the gang for too long Loved the Brothers saying "they were real fighters" during chapter 1, but Mac and Arthur seem to have a bit of a strong connection seeing how a cut voice line from the game after you bring Dutch his pipe tells us that the way Arthur really escaped the Blackwater Massacre was because Mac gave him his horse to escape saying something along the lines of "only one of us can survive" also another cut voice line from the game during the hunting mission with Hosea named "Exit Pursued by a Bruised Ego" Arthur would've mentioned something about how the gunslinger Jacket was originally Mac's before he sadly passed.
The Death of Mac Callender is told through Agent Milton in a way that sounds mostly like a combination of John's Death in RDR and Arthurs in RDR2 as John was shot multiple times by all kinds of weapons and standing there for about a minute before falling to his death whilst Arthurs "True Death" is known as the High Honor ending where he dies in a bit of a Merciful way as he views the sunrise for one last time and now if you look at Mac Callenders Death he was shot multiple times but he ends up running off only to be caught and given the sweet release of death for letting him bleed is looked upon as an agonizingly slow death so in the lake cut scene with Jack Milton was semi right on giving him a "merciful" death.
In conclusion, all this is one theory I thought of and found quite interesting due to what is so little known about Mac, and if RDR3 is a prequel like I hope it is we are sure to get a deeper dive into the Callender Boys whether they side or main characters.
(sorry for any spelling mistakes made and don't be scared to point it out for I will take it as advice, feedback, or any positive way I can.)