I've wondered ever since Carcosa, so I might as well ask during this last month we've got.

Father Mateo. William Yorick. Tommy Muldoon. Nathaniel Cho.

These four investigators all have the Warden trait. They're not all part of an organization (such as the Agency). As far as I know, they don't all have some sort of special destiny or supernatural power. The term implies a duty to protect something, but so does the entire Guardian class (and Mateo's not even a sub -Guardian). Gameplay-wise, they can all take low-level Blessed Guardian cards, but so can plenty of other investigators, including some far more Bless-themed ones (ie Sister Mary).

Is there something more that ties these men together? Some tie-in novel that explains (or implies) what a Warden is? Is FFG dropping hints to a future series of books in which the four investigators team up and become Wardens? Or am I just overlooking something obvious?

Good question. I asked about the trait thing in a Q and A last spring. How important are these traits? Are they going to start using them more in some way in game play? Like the Cursed trait. That seems like a pretty big deal. Several cards, some weapons and even a few Investigators are Cursed, but it doesn’t seem to really change anything in game play.
Warden, as you say are Protectors, but there is the implication that they also have some sort of ability to dampen the effects of the bad guys. They do all have abilities or access to cards that kind of bear that out.

In their spare time, they issue tickets for poor parking...

I was wondering if it had something to do with having a very specific area that they watch over. For Mateo, it would be the church. For Yorrick, it would be the graveyard. For Tommy, it would be the streets of his 'beat'. For Cho, similarly, it would be the streets of his 'beat' (his story indicates that he is 'patrolling' the streets). This is all speculation of course. But I don't think there's any deeper lore behind it than that. No mystical role as special 'warden' chosen one etc.

Lily is the only real chosen one.

The Oxford English dictionary defines warden as:

noun

a person responsible for the supervision of a particular place or thing or for ensuring that regulations associated with it are obeyed.

1. "the warden of a local nature reserve"