The Aztec Pantheon (the Teotl) claims there have been five previous Creations before this one. Some of their members claim to have existed throughout all five, and say that they are responsible for placing the current Sun God(s) in the sky, which is apparently a necessity if linear time is going to exist. Most other pantheons, and particularly the various sun gods, strongly protest this. The Aztec Pantheon has not forced the issue, but quietly go on believing that the various sun gods are all merely aspects of frail, old Nanahuatzin, the most humble god in the Aztec Pantheon, who sacrificed himself in the pyres of what would become the new Sun.
Often seeming to have created Themselves out of nothing, the Sun God(s) (Amun Ra/Belenus/Pelor/Amaunator/etc?) vanquished the great beast(s) that had arisen from the dark seas of primordial Chaos (Cipactli/Nu/Tharizdun/the Tarrasque/Tiamat/Leviathan?). The beast was torn to pieces in the battle that followed: thirteen Heavens were formed from his head, nine Underworlds were formed from his tail, and all the rest of the new Creation was formed from his body.
The Sky Father (Cronus/Ymir?) took clay from the Earth Mother and created the First People, the first mortals-- who would later achieve immortality, in the form of the Guardinals, and even a kind of divinity as the Animal Lords and gods of the Egyptian Pantheon. They were given a gift (a jar/a cedar box/something else, placed in the safekeeping of one of their number (Pandora/Seagull/depends who you ask), left over from a previous Creation, which, when opened, released the stars in the sky, and with them, all the ills of the world-- the first disease, that of evil itself, later to become embodied in the yugoloths.
Followers of the Aztec pantheon attribute Quetzalcoatl with taking pity on humans in a previous Creation (in which he also destroyed them for their hubris, scouring the world with arcane wind), which led to His recreating them in the current Sun (after retrieving their bones from the vault of the death god Mictlantecuhtli).
The race of Giants (the Titans, the Jotunns, the Fomorians, ‘monsters’ in general) were the first children born of an actual union between the Sky Father and Earth Mother (Uranus and Gaia?); they ruled Creation early on, before being overthrown by their own children (the Olympians, the Aesir, the Tuatha De Danann) and, for the most part, imprisoned in the underworld (Carceri, yes, but maybe other Lower Planes as well). Their weaker siblings, the lesser giants and monsters that are still around today, managed to hold on to power for a short while longer-- for instance the (lowercase) giants’ enslavement of the dwarves, which lasted for many years before Moradin appeared and liberated them.
The Ancient Baatorians-- pain elementals which survive in the present in the form of kytons, bladelings, nupperibo, and maugs-- were created to exemplify necessary evil. Not the selfish will-to-power of the baatezu, or the gratuitous slaughter of the tanar’ri, or the various shades of greed and pettiness demonstrated by the yugoloths, night hags, gehreleths and rakshasa, but evil toward a purpose, however regrettable, as selfless in its way as the crusades of the archons. The Ancient Baatorians were there to punish those who went astray in life, and to ensure that conflict and strife never ended. A steady stream of blood and sacrifice was necessary-- to grease the wheels of Mechanus, thus keeping the fires of the Sun Disc of Aten forever stoked. The Ancient Baatorians are the lock which keeps the Chained God chained, for if he were ever to be freed, the great beasts of old would reform his body from the current Creation, and all would be destroyed in the process.
Willing sacrifice is just as good for this purpose, and if the followers of good gods lay down their lives for cause or country, glory or a chance at paradise, all the better; if devils and demons go on killing each other for the rest of eternity, that’s fine as well. Blood is blood, and as far as the Aztec Pantheon is concerned, death always comes as an honour.
So that's one idea, or several of them. Finding ways to mesh the various pantheons, both from our world and others, finding ways to reconcile the contradictions, even if the reconciliations are just as messy if not more so-- I find that interesting. These are gods, after all, in a world that is shaped by belief. They can rewrite memory and thus history according to their whim, bilocate and appear in a multitude of forms. Who's to say their own memories of the past are any more reliable than those of mortals? Only they themselves.