Do you really believe that all rpgs contain equal levels of heteroglossia?
So, the problem is that we don't have a
measure of heteroglossia - largely because genre, theme, and style are things we don't have measures for.
So, using the Vampire example, one person can say that it does a bunch of stuff, including politics and cross-over urban fantasy.
But I'm here thinking that there's darned little daylight to be found between politics and cross-over urban fantasy - the crossover stuff is usually centered on the politics between the groups! What there is in crossover stuff that isn't politics is Supers or Romance. Supers is another duplication, and Romance doesn't have much direct support in WoD game rules.
The end result is that two reasonable people can look at a game, and disagree on the level of heteroglossia supported by it.
@Malmuria
The essential feature of games like Sorcerer, Apocalypse World, Monsterhearts, Cortex Heroic and Masks is that theme is something we discover and define through setup and active play.
Like this - I see this and think that Masks is thematically constrained, to use Campbell's own term.. Masks is designed to do teenage supers angst. And that's about it. It literally hands you a set of iconic figures for that in the character playbooks. Anything else requires a full rewrite of the playbooks, and when most of the genre/theme divers of a PbtA game are in the playbooks, that's basically saying that I have to write a new game. Heck, I looked at Masks and thought about doing Middle-age supers angst, and gave it up because I'd need all new playbooks.
Now, I can agree that the themes and style of
the engine are more open. The PbtA engine, the Cortex Prime engine, and so on, aren't nearly as constrained as any particular implementation of that engine. But then we shouldn't be comparing PbtA to D&D - we should be comparing PbtA to the d20 engine.
But even then, there are some constraints. For example, PbtA does not, as an engine, support tactical wargame style play well, no matter the playbooks.