Li Shenron
Legend
Is this rumor, or do we know for a fact this is planned? And do we know if it includes guidelines for mixing features from subclasses of two separate parent classes? (I'm bugged that, from what we've heard/seen, the druid lacks an animal companion option like the ranger has. )
Yes, it's confirmed but supposedly only with relationship with mixing subclasses of the same base class.
That's because it's easier, since some subclasses alter the base class following the same pattern (e.g. cleric domains, all of which grant domain spells, additional channeling options etc.), and because many subclass features just improve/expand features of the base class and are not applicable to other classes.
But on the other hand, it's clearly possible to also mix subclasses of different classes, only more difficult because you have to check each feature individually, if it's applicable to the other class as well. Also, it might become evident later than some class has more potent subclasses (not just in terms of number of levels worth of feature).
I don't think the guidelines will go this far, at most they will tell you that you can try. I don't know exactly what's in the Ranger Beastmaster subclass, but if its features are not built upon other Ranger abilities (that a Druid doesn't also have), I guess it's perfectly doable to splice the Beastmaster as a whole onto the Druid (just check the levels worth of features are the same, or adjust).