Yes, writers sit down all the time and start writing without any idea of what should be in the novel. No plan, no theme, no guide other than put words on the page hope you get Shakespeare. [emoji849]
And don't forget Shakespeare's motto: "But where do I draw the line?"
What is your wishlist of things that you might want included or believe should be included in the 6e PHB what would satisfy your standards of inclusivity?
I'm still not sure if this is the appropriate question to ask, because this question seems to presume a radically different sense of "inclusivity" than the OP's use of the word, and we should probably avoid any potential pitfalls that comes from equivocating between these two diverging senses.
FWIW, I don't think that saying "Hey, we should include at least include a fighter, cleric, rogue, and wizard in PHB 6e" actually engages the OP's premise regarding making 6e inclusive at all. IMHO, making 6e more inclusive is more about the intersection between the rules and flavor text of the game with how the game represents diverse genders, sexualities, ethnicities, cultures, disabilities, bodies, creeds, and so forth. In some regards, it is less about the list of PC archetypes we include and more about how we depict/write about the things we do choose to include. Simply including a shaman class, for example, is meaningless and antithetical to "inclusivity" if the game fiction frames it in a negative light or using harmful stereotypes. The orc has obviously been included in the 5e MM, but the issue of inclusivity is not in reference to its incorporation into the book but, rather, in reference to the rhetoric that frames it therein.