Hey all! 
I have actually played in several adventures of this magnitude (albeit in previous incarnations of D&D/AD&D). Although to be fair I don't think 100th-level in AD&D is the same thing as 100th-level in 3rd Edition. Something like Throne of Bloodstone would probably convert to around a 40th-level adventure in 3rd Edition D&D. 
However I think a 100th-level Adventure is certainly possible (in fact I'll probably have a stab at designing one in the near future). Although it probably throws up a number of issues unpalatable for most campaigns.
Firstly, there are not very many people who play in a campaign on this strata of power. Which means that for most it would merely be a oneshot with pregenerated characters. The less time people have invested in characters the less they empathise with them. Someone who has played that character up from 1st-level to 100th over 10 years is going to care more about it than another who rolled up a character 10 minutes ago.
Secondly, part of what makes events 'epic' are the far reaching consequences. If you don't plan on living with the aftermath, then again the whole things is going to lose much of its gravitas. Its difficult to make an omelette without breaking a few eggs.
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How about an adventure where the Phaerimm conquer your world and use an epic ritual to send it into the Far Realm where Pseudonatural Devastation Caterpillars start to cocoon it. The Players are drafted into a divine strike team brought together to reclaim the planet. 
However, first they must recruit the final member of the group, Sharkon, an intelligent animating sword once worshipped as a deity. A weapon which can cause the extinction of an entire race, if a paragon of said race is slain by the deadly weapon.
Sharkon resides in the Comet Citadel of the Elder God Damolh, whose frozen fortress is now policed by powerful star-metal constructs from the future, although some of the original guards, Xixecal, still wander the outer sanctums.
Sharkon has been imprisoned, imbedded within the carcass of the Elder God for eons, although after the fatal blow had been struck Sharkon found he was unable to extricate himself from the now corpse of his enemy. Over the millenia Sharkons delusions manifested themselves, haunting the inner sanctum in the shapes of undead of the long forgotten races whose existence were ended by the blade.
The team then must breach the borders of the Far Realm guarded by World Flayers and penetrate into the depths of madness to reach the Twisted Tower of the Phaerimm Overmind and with its destruction cause the extinction of the entire race. However the ruler of the Phaerimm, as his last act, summons an Infinitaur (apparently the twisted tower was one of the beasts horns).
The players must then destroy the cocoon around the planet. However, they then find the planet itself has become self aware, and whats worse, insane. Before they can finally unravel the ritual of the Phaerimm, they must confront and defeat the planets id. If they are too late, the planet may have already hatched into the unspeakable mind moth which then multiplies by its proximity to sentient creatures, so you soon face a swarm of the titanic lepidoptera.
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Anyway, thats just off the top of my head, the idea probably needs some work to it.  
 
Prologue: Planet gets stolen by the Phaerimm.
Part 1: Free Sharkon from the Comet Citadel.
Part 2: Destroy the Phaerimm Overmind in the Twisted Tower.
Part 3: "Its alive, ALIVE!"
Epilogue: 
- Freeing Sharkon eventually causes the Elder God, to rejuvenate, a two-headed monstousity searching for its third head which is now a demilich. The keys to the Elder Gods ultimate destruction lies within the gems of this massive demilich, each of which is a demiplane housing a fragment of the artifact which can bring about the demise of the cosmic deity.