In addition to Mythic+ changes announced in Blizzard's Dungeon Level Squish blog post, it looks like there will be another +39 item level increase between Dragonflight Season 3 and Season 4! Using these numbers, we can extrapolate that Normal Fated raids should be item level 493-502, Heroic between 506-515, and Mythic from 519-528.
*Note the greater increase for Heroic dungeons and Mythic 0-10 in accordance with the key level squish.
Each new season, item levels increase across the board to help maintain a sense of power growth and progression, soft nerfing previous content and empowering players to take on new content. This affects nearly all facets of the game, from open world rewards, to PvP, dungeons, and raids. Prior to Season 3, the increase between tiers has been +26 item levels between the same type of content, such as normal difficulty Vault of the Incarnates to Normal difficulty Aberrus. This changed in Season 3, growing to a +39 item level increase, widening the gap to give players more meaningful upgrades at the cost of less immediate relative power within the new season of content.