One way to use the theoparts Idea with a twist, one which relies on having a stable playing group is
A) Run a standard campaign in one world (say forgotten realms - princes of apocalypse or out of the abyss) but add in a bunch of magic items that can be combined to make an uber item around the time the campaign ends ie a ring of warmth, a wand of binding and a broach of shielding that can be fitted together with an unusual black diamond to make a thing that takes up one attunement slot for all powers and additionally evards tentacles & disintegrates 2/long rest; then
B) Run another campaign in another realm say a dragonlance one where the new party also finds a bunch of items that can fit together to make one uber item, again requiring a black diamond; then finally
C) Run your final campaign - in Greyhawk where the world is suffering from nightmares and far realms shenanigans leading at some point the party trying to stop the bad guys from assembling the third of three theoparts because the first two have been accidentally assembled somewhere else in the multiverse and if the third is assembled the three uber items (or theoparts) can be used to unchain Tharizdun which will end the multiverses (in part because Asmodeus “binding” ability will fail, fiendish contracts will have no force legitimacy, leading to devils abandoning the blood war and siding with demons in destroying the multiverse )
Apart from introducing the uber magic item in each of the first two campaigns you don’t really need to do anything. But if you do it, it really is playing the long game.