I have two suggestions in hand that I've been thinking about, and I wanted to explain my rationale for not doing either of them -- at least not for now.
NPCs to be documented in terms of "where to find"
-- Professor Paul1290
A description somewhere of the who/what/where/when/why of getting new people into your settlement, how that affects you, and why you should even care. Right now it's one of those 'explore as you go' things and its REALLY annoying, unlike other things that you explore and go OH NEAT! I should look forward to finding a new NPC, not look at him and go "Why the heck do I care about you?!"
-- Wanderer
In terms of Professor's original comment, the need for that was because those missions were so darn rare. But as of a few versions back, they are now way more common. So people are just naturally going to be finding them in caves and buildings. And to some extent, I'm not sure how people were missing them in the caves in the first place, since you have to go down there to find gems and then will wind up seeing the missions. So in terms of documenting where to find NPCs... I think that's already pretty much worked out.
Then thinking about Wanderer's suggestion. On the surface I like it -- both of these suggestions come from really sharp commenters, I should say, so this isn't a commentary on them in any way. However, the thing is... isn't that suggestion getting a bit cart before the horse?
When you meet a person in the wild and they need to be saved from death, is the first reaction really "what's in it for me?" If so, my inclination is to let the player just let the NPC die. Then later if they realized they actually wanted that NPC around, that's a little subtle life lesson.
Not that I'm trying to be preachy here, and of course it's just a game and again this is really NOT a comment on Wanderer specifically in any way. But it seems to me that in the context of games, you can assume that rescuing somebody leads to something good, even if you don't know what.
And that's what I mean about the cart before the horse. It's an exploration game. We sure do tell you a lot of things in advance, but at the same time some things are better left for you to find. That's why I've been so back and forth about a monster list included in the encyclopedia parts of the game. It's a lot more fun to just go out and find that sort of thing in particular, is my personal belief. The frustration comes when you need X and you have no idea where to find it in a giant world, and that's where the wiki comes in for Minecraft and the in-game encyclopedia comes in for us.
By the time you actually need NPCs -- aka, finding guardian power scrolls -- it tells you that you need them. By then you've probably passed some NPC rescue missions, and maybe even rescued a few. So you probably have a pretty good idea what to do to go find some more NPCs.
I was talking about the monsters list thing in particular with a reviewer a few weeks back and asked his professional opinion; his response was that he felt like it's definitely possible to tell too much, but that we seem to be striking a good balance at the moment. This is somebody whose opinion I really respect, so I've been reluctant to stray too far from that line, so to speak. Anyway, that's my rationale.