I just hope they are less predictable. The mages guild quest is the perfect example: you have one not-so-mysterious enemy the whole time. You basically watch a guild of professional scholars run around like a pack of bewildered rats, then all you have to do is fight the guy anyway. It's classic dull.
I would rather more Thieves Guilds and Dark Brotherhoods where personal stories, unpredictable endings, and plot switches are common. I don't want to be able to see what's coming so easily from the very beginning.
Also, the rewards. Guild rewards have been sooo terrible for such a long time. Thieves guild was the best guild reward they've ever done. The rest were crap. I used to just make a house rule that Frostcrag Spire and Battlehorn were actually the quest rewards for the Mage/Fighter guilds. I wouldn't use those homes unless I finished the guild line.
So... please, I would really love some Guildmaster benefits that mean something.
^ This.
Make them longer por favor, and much less predictable. Like OP said, the DB was my absolute favorite quest line, but was still predictable. The only way I could make it harder was the first time through. I didn't think (in my own arrogance) to ask my Brothers & Sisters how to get through each contract so it was much more difficult having to figure it out the first time myself. But that also made it much more fun. lol
And maybe a side-quest line where it overlaps into another Guild? Maybe have it where the DB and the Morag Tong are at odds, but you have the choice of joining either one, maybe switch sides in the middle double-agent style.
And please, please, please make it worth my time to actually
be the Guildmaster. Give me something good.