Here's what I aome up with.
TOL FALAS/THE MASTER
4 possible ways to make this more narratively interesting...
The phylactery-lichy way
The Master has a vault where he keeps some kind of phylactery/amulet/magical artifact which lets him resurrect once. Yep, phylacteries are a lich thing, but the Master is not simply a powerful vampire - he's a powerful sorcerer, and thus...
For people who choose to ignore the vault, this doesn't change much, except that upon killing the Master the 1st time, you'd get a message like 'You feel he is not truly gone', and he would respawn in this vault and immediately start hunting for you.
For those who manage to find the vault - perhaps through following some hints given in the lore...
It's guarded by a sub boss, say some kind of vampiric abomination.
They would be strong, but not as much as the Master(or else, what's the point of killing them instead of killing the Master a 2nd time?).
You kill this guardian, then grab the phylactery and destroy it. After that, when you go fight the Master...they won't resurrect.
The questy-romantic way
This is a bit different as it involves stuff outside the dungeon.
By reading the lore bits in the Tower, you might find out the Master had some kind of mortal lover/friend/whatever in life, hundreds of years ago.
By adventuring outside the dungeon, you might eventually have some kind of encounter/sub-quest, at the end of which you get a piece of jewelry which belonged to them.
Maybe it's a family heirloom and you'd get it from one of that person's descendants.
You equip it - it would provide no benefits, and it would take an equip slot so that's sorta like a penalty - and when the Master first sees you, they could be stunned for 2-3 turns, and then confused for a while(as they are overwhelmed by memories of their mortal lives or whatever).
Basically you'd surprise him and get a minor starting advantage .
The backstabbing-treacherous way
On the upper levels of the tower, you face an unique - something like 'The Apprentice'(yeah, needs a better name), basically the Master's most powerful vampire minion/second in command.
While strong(he/she would be a buffed master vampire), they are still not a match for the Master.
Not by themselves at least.
At first they would just attack you, but if you defeat but not outright kill them - by bringing them below 25% or 20% life - they will call for a truce by starting a chat with you.
They will offer to help you kill the Master.
If you accept, they become an ally and will follow you to the upper levels of the tower to help defeating the Master.
However, after the Master goes down permanently...there should be a chance based on how wounded you are compared to them and perhaps your luck that they betray their part of the deal and just turn on you.
This should not be guaranteed, again - if at the end of the fight you are still strong and mostly unwounded, while the Apprentice isn't, they'd realize they wouldn't have much of a chance and so just let you go on your way(narratively, they might want to grab the Master artifacts for themselves, but let's ignore that.).
If on the other hand you are severely wounded and they are in better shape, well...you'll make a tasty post-fight refreshing snack
The Blood of Life
This is an afterthought and so might feel badly designed but - as the Master mentions wanting to be able to walk under the sun(well, to conquer the areas under the sun..he is not a nice guy), what if you could bring him the Blood of Life and have that do something to him?
He'd grab the Blood, drink it, and turn into a mortal...a mortal but still awesomely powerful sorcerer however. He'd weakened and wouldn't resurrect but still should be a tough fight.
Oh, and even if he's grateful, he'd still attack you - you're too much of a threat to him and well, you ARE in his domain.
Mechanically: you're basically giving up YOUR resurrection from the Blood of Life to negate his(and weaken him a bit).
ELVEN RUINS/GREATER MUMMY LORD
What if you could prove yourself worthy of wielding Ringil(which of course would have its name changed in 14, but...)without defeating the Mummy lord?
Yes, it's an heirloom of his, but he doesn't have much of an use for it now - he's powerful enough to defeat any would be tomb looters without it - and if not malicious he could understand your need for the sword to fight evil outside.
Some ideas to prove yourself 'worthy' / show your respect:
1)Enter the ruins wearing some kind of holy amulet/clothes
2)Don't fight the mummies - if one attacks you, you should simply resist their attacks for a few turns and they'll turn neutral. Perhaps following 1/wearing some kind of holy attire would make them neutral by default.
On the other hand, killing the vermin inside the ruins would be ok(hell, the mummies might even appreciate it), unless you want to be extreme and enforce a 'no bloodshed or fighting at all'rule.
3)Do NOT loot the place. Perhaps autolooting gold on the ground should be disabled by default.
The first time you attempt to pick up something manually, you get a 'You're not sure that would be the right thing to do...' and it fails. The 2nd time, it works fine and you stop getting the messages(and autolooting gold is enabled again).
Anyway, let's say you follow all of these rules and get to the Mummy Lord alive.
You'd talk.
At first they'd be annoyed at you disturbing their rest, however after realizing you've been respectul enough, they would understand your mission/need for some of his treasure and hand you over Ringil.
Or maybe, if we're feeling generous, he could hand over a generated randart which is supposed to be useful for your class.
They'd have quite a lot of burial treasure, and so would give you the ONE piece they feel is most useful.
As for how the player would know about this: it should be mentioned by some priest-like/archeologist/sage character(someone in chat suggested from the East, which might work if their lore is generic enough, as they might not know about this ruin in particolar), or found in some lore snippets.
Maybe it could be a distant descendant of the mummy lord? Or not so distant, as he's an elf so he could have a (grand)daughter/son still around in Angolwen...Perhaps contacting the descendant could be part of a side-quest given by the Mummy Lord to prove yourself worthy?
CARN DUM/RANTHA
This isn't really a side quest, but just a minor idea I had.
Instead of making Rantha drop stuff, why not have a 'hoard' vault/lair? It would look cool, a lot of gold and treasure on the ground. Feels a bit more atmospheric than just grabbing the stuff from the corpse.
Maybe there could be a sub-boss in the lair..Rantha's Consort/Mate. Or just a few extra-powerful dragon hatchlings.
Rantha wouldn't necessarily be found inside the lair - might be out hunting. However, however the moment you grab something from the the hoard...they'd know it and would start going for you, perhaps with some kind of rage/fury effect.
