After you collect all the keystone pieces and assemble it, you have 2 options:
Delete your character, lose everything, and get a lackluster “cutscene” of leaving the Exiled Lands, or
Store the keystone in a box and forget about it.
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Please give the main quest meaning!
Character deletion will probably always be the actual ending. So let us have a choice what to do with the keystone other than storing it away.
Maybe a new ability to free NPC’s. Say you have a level 20 thrall, maybe a named one, and you free them (= remove their bracelet). An act of benevolence that will reward you: Next time you get the same named thrall (see a brilliant connection to my 20 Dalinsias problem wink wink), they will have boosted stats, or even better: new perks. Or both.
You can think of it as technically higher tiers. But better! Because… story tied and… I mean, it’s just more interesting than simply adding higher tiers arbitrarily.
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Freeing thralls with the specifics I’ve described, is just 1 of many ways the main quest could become meaningful.
As much as i currently fancy this particular idea, the main goal is to simply give some meaning to the main quest and do something with the keystone.
Hu! Interesting idea. I agree the main quest is a bit weird: “Congrats for beating the game, now we will delete your character.”
To give another idea: Why not free Razma of Shem? She is in some way the other main character of the story, and freeing her would be an act of mercy. The Exile instead accepts his or her terrible* fate to remain in the Exiled Lands, similar to how Planescape: Torment ends.
What would be a good reward for the player character? A cool title, skin or other cosmetic thingy would be fine for many, I suppose. If one wants to add something mechanical, the list of possible ideas is long. She could reveal some secret or treasure she learned in her time as the Witch Queen, or share some cultural Shemite knowledge. That might be a a stat boost (e. g. +100 life), a aura that boosts thrall damage (e. g. +10% due to the Exile’s good reputation), maybe a new building (e. g. decadent Shemite bed) or crafting station or recipe (e. g. a Shemite armor), or - a personal favorite - revealing where the f… Telith’s Ghost can be found.
To give a wild idea: She could teach us the Dagon religion. As Witch Queen, she was or had the memories of a priestess of Dagon.
The Dagon religion could, for example, give underwater and swim speed potions, a religious feast made from fish, a poison and a trident weapon.