Having recently read LOTR and having taken note of every single mention or reference to Rangers specifically, I'll mention that Rangers as Tolkien conceptualized them fall into two camps:
The Dunedain: These folk's main traits were your classic explorer type Ranger stuff, and notable among the things they do beyond just being capable warriors were the use of runes to leave messages and a skill with Herbs for healing purposes (that is separate from Aragorns destined skill with healing, mind). They're also implied as some of the greatest hunters and trackers, with Aragorn being directly named as the greatest.
Faramir's Company: these folk are more the stealthy types, and act mainly as a military unit in this capacity. They're essentially guerilla fighters operating in the Gondorian border regions.
Both taken together obviously share this trait of operating in border lands (Ranger as profession), but they aren't strictly limited to this (Ranger as Adventurers).
Likewise, they both share a clear flexibility to operate across a spectrum of Warrior types from the heavily armored cavalry to a standard Knight-on-foot to a stealthy, lightly armored fighter.
So taken altogether, one could synthesize the idea of a Ranger in terms of an RPG class as being martials defined by flexibility moreso than other martials, with a clear focus on either individual or small unit tactics and a strong familarity with and skill in the Wilds, to the point of being able to wield the Wild, so to speak, in some fashion.
Which all more or less confirmed my personal take on what the Ranger should be, to be frank. I'm designing mine as an AOE martial with a focus on Healing and Stealth, with a core mechanic based in a much more indepth version of Natural Explorer, which will provide not just basic travel and exploration benefits, but will also confer abilities like sending messages into the wind, provoking stampedes (even of dragons), and enhanced effectiveness against hordes or gigantic creatures.
They will be capable even by themselves, and will hit all the thematic marks LOTR style rangers bring to the table, with subclasses exploring some later variants like the Ranger's Apprentice or the more direct "monster slayer" takes like your Geralts and what not.