As I've already noted a few times, I'm pretty well soured on the idea of skill systems in D&D. I won't recapitulate my case against skills here; if you want to see that, go here.
But there are still a couple of cases where I would like characters to be able to do certain things, which don't directly pertain to fighting, magic use, stealth and guile, or religion. Things that are frequently useful in an adventuring career, that aren't so universal or easily learned as to work well as things that everybody knows, and that can't be modeled at the table through role playing or player/DM dialogue.
The first one that comes to mind, that inspired this post in the first place, is tracking. Any fool can look at tracks in the mud, but reading subtle signs to determine whether something passed through, what it was, how recently, its physical condition, its speed, etc. requires some uncommon expertise. Some editions add an entire character class (the ranger) to provide for tracking, but the class comes with an additional suite of abilities and restrictions.
Healing/herbalism is another. Probably almost everybody in a medieval setting knows at least a little about how to bind a wound, but to do it well, and to treat diseases, poisons, and severe injuries requires more specialized knowledge and experience. Again, D&D foists almost the entire responsibility for this important role onto a single class, the cleric.
I don't see any good reason why tracking should be limited to a particular fighter sub-class, nor why healing should be the sole province of the religious. I also don't like the idea of giving those abilities to everybody.
A possible fix: Meta-classes that bolt on to any standard class, adding the relevant abilities and a premium to XP progression - say, 500 XP added to the base XP required for level advancement. Taking a meta-class might require the player to justify his or her character concept to the DM, but otherwise they could be combined with any of the standard adventuring classes. Fighters who are battlefield medics and witch-hunting clerics who are expert trackers are possible this way.
This way, a character actually has to pay a price to obtain those abilities. There's a genuine trade-off, and it's an ongoing one, instead of a one-off cost for a permanent benefit. A thief who chooses to be a tracker too is going to be less effective as a thief than a "pure" thief with the same XP total.
Since the cost is ongoing, and since I'm putting these abilities in the context of class advancement, they should scale with level, starting out very limited and increasing in usefulness and reliability. That means some objective mechanic for resolving those abilities. I'm thinking of a 2d6 roll, similar to a cleric's turn undead ability. How exactly that might be structured is a little beyond the scope of these musings, but I'll be giving it some thought to see if I can cobble together a workable system. (If anybody else wants to tackle it, feel free to beat me to the punch.)
I suppose in theory you could have characters who were ONLY trackers or healers, but meta-classes don't provide for advancements in hit dice, attack rolls, saving throws, etc., so in all respects but the special ability of the meta-class that character would be a 0-level normal man or woman. Come to think of it, that might actually be desirable in NPCs.