I haven't fully decided yet, if they're not granted specifically by a feature on the levelling table, I was considering the notion of working it into their spell preparing directly. Maybe not as a mechanic that specifically serves this idea, but a rising tide that lifts all boats. I think I mentioned it in a previous post, the idea that some of their spells can be "hot-swapped". Just for example maybe, "You can exchange a number of spells from your list of prepared spells equal to your Wisdom modifier over the course of a short rest, or by meditating for 10 minutes (as if ritually casting a spell)"? Something that could suggest, Rangers aren't as absolutely rigid as other magic users, they can better adapt as the situation changes. As I say, not entirely sure yet, need to think on it some more.