Gulliver's Travels is political/societal satire, this we know, but I'm sure you'll agree it's just as interesting and entertaining to envisage it as straight fantasy. One of my favourite episodes from the book is Gulliver's sinister encounter with the Struldbrugs, which is available online here. A Struldbrug is a rare citizen of Luggnagg who was born with eternal life, but not eternal youth, and is thus condemned to old age unto infinity:
[T]hey commonly acted like mortals till about thirty years old; after which, by degrees, they grew melancholy and dejected, increasing in both till they came to fourscore [...] When they came to fourscore years, which is reckoned the extremity of living in this country, they had not only all the follies and infirmities of other old men, but many more which arose from the dreadful prospect of never dying. They were not only opinionative, peevish, covetous, morose, vain, talkative, but incapable of friendship, and dead to all natural affection, which never descended below their grandchildren. Envy and impotent desires are their prevailing passions.
But those objects against which their envy seems principally directed, are the vices of the younger sort and the deaths of the old. By reflecting on the former, they find themselves cut off from all possibility of pleasure; and whenever they see a funeral, they lament and repine that others have gone to a harbour of rest to which they themselves never can hope to arrive. They have no remembrance of anything but what they learned and observed in their youth and middle-age, and even that is very imperfect; and for the truth or particulars of any fact, it is safer to depend on common tradition, than upon their best recollections. The least miserable among them appear to be those who turn to dotage, and entirely lose their memories; these meet with more pity and assistance, because they want many bad qualities which abound in others.
As soon as they have completed the term of eighty years, they are looked on as dead in law; their heirs immediately succeed to their estates; only a small pittance is reserved for their support; and the poor ones are maintained at the public charge. After that period, they are held incapable of any employment of trust or profit; they cannot purchase lands, or take leases; neither are they allowed to be witnesses in any cause, either civil or criminal, not even for the decision of meers and bounds.
At ninety, they lose their teeth and hair; they have at that age no distinction of taste, but eat and drink whatever they can get, without relish or appetite. The diseases they were subject to still continue, without increasing or diminishing. In talking, they forget the common appellation of things, and the names of persons, even of those who are their nearest friends and relations. For the same reason, they never can amuse themselves with reading, because their memory will not serve to carry them from the beginning of a sentence to the end; and by this defect, they are deprived of the only entertainment whereof they might otherwise be capable.
The most compelling and interesting of Gulliver's comments comes at the end of the chapter:
I could not but agree, that the laws of this kingdom relative to the STRULDBRUGS were founded upon the strongest reasons, and such as any other country would be under the necessity of enacting, in the like circumstances. Otherwise, as avarice is the necessary consequence of old age, those immortals would in time become proprietors of the whole nation, and engross the civil power, which, for want of abilities to manage, must end in the ruin of the public.
It makes me want to imagine a society in which Struldbrugs or those like them are indeed the rulers; without the laws which legally declare them to be dead at 80, they have gradually accrued the wealth of the entire nation and now use it only to dispose of their envious, avaricious and senile whims. You'd obviously have to tone down the effects of their malaise a little (for example, their short and long term memory loss) to make it at least vaguely believable that the Struldbrugs could have the wherewithal to do anything at all, no matter how capricious.
It sounds a bit like something that might be found in Planescape, somewhere in the Outlands maybe. This is reinforced by the fact that Planescape contains two sects who are a little like the Struldbrugs: the Incantifers, incredibly ancient and ever-living wanderers who exist only to accumulate all the magical knowledge of the multiverse; and the Prolongers, who steal the life-force of others to perpetuate their own. Perhaps the two could be merged into a society of venile old sages whose only aim is to accumulate magical power, and who in order to do so drain other peoples' life forces to prolong their own existence indefinitely. They are senile and governed almost entirely by whim, but have so much sheer magical might that they can never be supplanted.
Except maybe by a group of intrepid adventurers, of course.