Can't really say what they might decide to do, based on the amount of people that want the full sandbox experience from the get go, there's a chance of that being given to new players (I can definitely see that being the case if they reuse a lot of assets, with at least those reused ones probably being in sandbox from the start).
Cross platform accounts is pretty unlikely, though on the same brand (PS4 to PS5 for example) they do share an online store/account, so there might be potential to do something with that (Steam's the only one I've seen it done for though).