So, I've completed all of the Thieves' Guild main quests (Completed "Darkness Returns"), but I haven't completed any of the city influence quests.

Anyway, Karliah hasn't returned to Nightingale Hall yet. Do I have to become guildmaster before she'll return?

Not AFAIK.... I just completed this (without the city influence quests - in other words, that toon is NOT Guildmaster, and probably will never be) a few days ago, and Karliah did exactly what she's supposed to do. Some folks have bugs with this, not sure why or what to do to fix....

A fix would be really nice

Yes, it would. If you do a search, you'll see just how many people have had this issue. And how many have not. And that there doesn't seem to BE a fix.

Even reloading?

The whole Thieves Guild quest line is a bug ridden mess. She is supposed to return to Nightingale hall but she gets stuck either in Snow Veil Sanctum or inn at Winterhold. She won't return even if you become guildmaster.

Well, not always. When it does happen, it's a bummer.

Isn't she in Nightengale Hall once you complete the Pilgrim's Path quest? Also, after completing the City Influence quests she shows up at the Thieves Guild as you're unenthusiastically made Guild Master... and Vex continues to speak to you like you're a worm.

At any rate, is there any good reason to ever go back to Nightengale Hall once you've taken back the Skeleton Key? Is the OP looking for her to be a follower or is she supposed to give you extra Nightengale quests? If not, why care?

This questline has glitched on me every single time and I've done it a few times. Nothing gamebreaking though, usualy just little bugs like that. I'm lucky with these things it seems.

Well, then! I feel VERY lucky - I've done it once (and that will be the LAST time I do it) and it worked perfectly! Oh hey, maybe I should find a casino?

Even by forcing her to appear in the right spot via console, she will return to Snowveil Sanctum or the basemant of the Inn in Winterhold.

Her script is bugged and it always places her in one of these two locations even if she's supposed to be in the guild or in the nightingale hall.

I've encountered her approaching Winterhold. Sure enough, she heads for the basemant. However, I have not confirmed this on my latest character.

I am meant to meet her at the Nightingale Hall and she isn't there. Should I go and search the basemant in Winterhold then?