I think that complex NPC follower AI is something that the industry desperately needs at some point. I think that Bare Mettle have loads to do already, so I'm not imply that it has to go in Sui Generis, but I can see it being the future of RPGs...
Suppose you've got a follower that happens to have a relatively short attention span. Suppose you're follower is quite happy to follow you - perhaps he's indebted to you, or you've paid him as a mercenary or something. However, one day you're running down the street, only to turn around and he's gone! Where's he gone??? He's got distracted, and gone into a passing shop or bar!!! Idiot. When you find him, you'll show him what for!!! Lousy good-for-nothing useless follower!!! So you go and find him, and he's really apologetic (or arrogant, or blissfully ignorant, depending on his personality and what he already thinks of you). So you decide to shout at him, or hit him, or something. Either you accept his apology --- he's promised to try and stay more focused in future, and anyway you need him as cannon fodder --- or you sack him, having decided that you'd be better off trying to find a more loyal and reliable companion.
This sort of mechanic would be much better than those used in existing games. For example, take Skyrim - apart from quest-specific followers, you choose your follower based on... well, nothing, really!!! Personal preference of what they look and sound like? How often they seem to die? Whether they've got a big axe or a dagger?
Basically, I'm fed up of having NPC followers that you've got absolutely no personal attachment to, don't have any personalities, and whether or not you sack/kill them and get a new follower seems like an arbitrary decision.
Of course, the other approach is the Bioware one, whereby all companions are completely pre-programmed with specific personalities and dialog options. Needless to say, that is far from a perfect solution.
I call for reform!!!