Totally agree! There are entire games that are sold based on that one game mode!
(Rockstar, take notes)
I've seen a few Maxim Wagons in Free roam, where people get them from bounties. They aren't as big a problem as you might think, as they are kinda impractical for hunting small, fast moving targets like people. And yeah, I don't think that a full blown Maxim won't be allowed, If it were, it'd have to give the player a massive Debuff like Heavily Slowing their movement speed (Along with the Armor Debuff), and maybe they would have to set it up in order to fire. Like the player would have deploy a tripod in order to fire.
Yes, I understand that the Juggernaut would have to carry 150+ pounds (including armor) but I mean a game that lets you Chug a moonshine to take more bullets could probably let that slide.
It was just an idea though.