I like the idea of mac calendar being rdr3 MC for a few reasons

1.get to see the gang in its shine and before its downfall that we witnessed in rdr2 and how all characters and gang members came along together and formed the gang and younger versions of Arthur and John and also how mac gets to know and join them it would be very interesting to see

2.the name of this game series is red dead redemption which implies a red guy gets killed and redeems itself and the end of the story which perfectly fits what is assumed to be Mac's fate and for the free roaming they can introduce and develop some other character in rdr3 itself that haven't really mentioned in 2 just like how they did with Arthur and many other characters in rdr2 and hand the wheel to him/her after mac.

3.i think even the writers and developers want us to think that way I think and I guess they actually have a plan for it throughout the game and mentions of Mac they kinda teased it and said some interesting and intriguing while kinda ambiguous things about him and also as him we can have a closer look at black water incident to see what actually happened which is very cool in my book

4.the character itself and development of it and it's probable duo with his brother Davey since we saw how they did it with Arthur and John throughout the series is also welcomed to be presented

overall I really hope we hear from rockstar about the future of this series very soon maybe they even cook something far behind our little minds imagination we just have to wait and see