Adoptable NPCs
Granting that most games don’t allow multiple PCs, granting that Ars Magica already has this model a little bit… I don’t see why more games don’t allow players to adopt particularly appealing NPCs.
Such a trick would, for example, ease the process of mopping up after character death: the player just picks up the already-adopted NPC and life goes on. It also gives more scope for roleplay, I think, because PCs have more people to interact with. Anything that eases drain on the GM’s time and energy is generally good, yes?
Obviously some NPCs are unadoptable for reasons of plot spoilage, but that needn’t invalidate the entire concept. Because, you know, there’s always that one NPC who came out of nowhere that everybody likes and one player loves… why not keep the NPC around?
What I prefer to NPC adoption, actually, is the arrangement I see in a fair few PBeMs as well as the Dragonhunt: NPCs who aren’t obviously plot spoilers live in limbo, free for anyone’s use. This gives so much more scope for thoughtful roleplay (and of course fluff) it’s not even funny.
I am of the selfish opinion that I’ve enriched a few Dragonhunt NPCs by playing around with them. Perhaps not the better-established ones like Sabra Congreve; but I’ll take credit for Sabine and Clara von Adler. (A previous Dragonhunt variant mentioned the analogue to Sabine in the tired old “time to marry her off!” chestnut. I don’t think Alan would dare shuffle off Sabine herself in that fashion. She’s too much of a person by now—besides, Aryk has an irremediable crush on her.)
Some caveats: If retcons are irretrievably evil in your game, then a Limbo full of NPCs will not work. We all slip up now and again. And players of course have to take extra care not to contravene established knowledge about and behavior of NPCs. While I felt fine, for example, upping Talos Clybourne’s respect for Renate over the course of their conversation (I thought she’d earned it, on the whole; she went in prepared and she presented herself well), I certainly didn’t turn him into a mindless von Adler flunkie. The guy is clearly an independent thinker, and I made sure he stayed that way; he didn’t so much as openly sign on to her plans.
For this to work, it helps if GMs signal NPCs’ opinions on matters dear to the PCs’ hearts, as well as attitudes toward the PCs themselves. Again, sometimes this isn’t feasible—but when it is, it helps.