I don't know any good huge story/quest mod for BG2, just some smaller ones:
G3 Anniversary mod, Dungeon Crawl, Unfinished Business (excluding buggy Kidnapping of Boo and Restored Minor Dialogues components) and Quest Pack (excluding buggy Additional Shadow Thieves Content and Revised Hell Trials components, it also has an AI component but it's just a mild rise of difficulty level).
Well, for story purposes I usually install some NPC mods, but I'm not sure if that's what you're looking for, because the story is told mostly via PC-NPC dialogues but they usually come with a quest of two.
Those best written, bug - free and with both SoA and ToB content are Angelo, Kivan and Xan imo. Another good thing about these 3 is that all of them made an appearance in the first game so having them in the party ties the plot nicely together, if you're continuing from BG1. Kivan and Xan have rather extensive dialogues with PC, some quest content and talk about the first game's events a lot. Angelo is not that talkative but has a really cool personality, some quests and since he was Sarevok's henchman in BG1, he makes ToB more interesting if you're having Sarevok in your party.
The only mod, other than Ascension and these NPC which significantly enhanches ToB, is Longer Road which gives you the opportunity to have Irenicus in your party. It adds a lot of dialogues and having him along is basically one huge quest. And he has a lot of agenda in Watcher's Keep (so rather don't go there in SoA). He's a bit overpowered but other than that, it's an excellent mod.
For some additional banters between original NPCs, Banter Pack works well, Crossmod Banter Pack gives some talks between Kivan and Xan, and if you're going to romance one of vanilla characters, install Flirt Pack for additional fun.
For some funny ocassional diversion I'd install Romantic Encounters as well, some of the encounters are seriously hilarious.
Also Weimer's Item Upgrade Mod gives Cromwell and Cespenar a lot of new things to do and you get the better chance to beat Ascension with this mod installed
![Smile :) :)](data:image/gif;base64,R0lGODlhAQABAIAAAAAAAP///yH5BAEAAAAALAAAAAABAAEAAAIBRAA7)
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One Pixel Production art pack takes care of the horrible BG2 avatars and such.
Well, that's just about it.
Here's recommended install order for the mentioned mods:
BG2 Fixpack
Ascension
Unfinished Business (stay away from those buggy components)
Quest Pack (stay away from those buggy components)
G3 Anniversary
Longer Road
Dungeon Crawl
Romantic Encounters
Xan (also has flaming swords animation component from 1pp which is cool)
Angelo
Kivan
Banter Pack
Crossmod Banter
NPC Flirts
Item Upgrade
One Pixel Productions (v2)
BG2 Tweaks
All mods can be found at one of these sites:
http://www.pocketplane.net/mambo/
http://www.gibberlings3.net/
http://www.spellholdstudios.net/
http://weidu.org/main.html
Imho, such setup shouldn't dissapoint you.
P.S.: If you'd ever decide to try some full scale tactical mod, don't bother with anything other than Sword Coast Stratagems.