Sure but I think the original concern was: what do you do when the DM and players all want to use different modules? A lot of people say 'the gaming group should decide' as if each group was a hivemind and the major divisions are between my group and your group, which maybe true but I think that the real problematic differences are those within groups. That's part of the social contract I was referring to.That's to say: if the rules are modular, the gaming group decides on which "rules modules" to employ at their table. I don't think you can apply "rules modules" on a per-player basis, aside from having different complexities of character builds to choose from which remain balanced with respect to one another.