Maintenance for the week of March 25:
• [COMPLETE] ESO Store and Account System for maintenance – March 28, 9:00AM EDT (13:00 UTC) - 12:00PM EDT (16:00 UTC)
It's safe to assume there will be cross progression between XONE and XSX, and there will also be cross progression between PS4 and PS5. There's also already cross progression between PC and Stadia.
Cross progression beyond that would probably require major server changes, and would also cause all sorts of character naming, guild naming, and trader conflicts.
Cross progression with PC runs into the issue of Add-ons. Either everyone would have to accept that PC cross-users have a leg up over solely console players, or we'd all end up with no addons or a tightly curated list of approved add-ons.
The latter would quite possibly be a deal-breaker for a number of PC players, since ZOS fully intended PC players to modify the game with add-ons from the beginning when the game was only available on PC.