"Steam Crossplay" isn't a thing as far as I know, until and unless Valve decides otherwise.
If, by that, you meant "on PC we used to need IP addresses until Steam MP was a thing so that made it way easier"... then that is the long term plan, more or less, but it likely won't be just like Steam MP, because there is no one "central system" in regards to crossplay on non-server-side games.
Initially (Phase One), IP's will be required - except for local play, where it should be pretty seamless, like it is on PC+Mobile now.
Again, assuming first parties (Sony/MSFT/Nintendo, etc) agree.
Phase Two will aim to make things easier and not rely on IPs. Would be best to save those details and questions for another day though, as we said in the OP. We will fill you in fully on the plan when we get closer... for now though, 1.4.5 is the focus.
No new classes, sorry.
We would not do this for a host of reasons. Primarily being that Crossplay by itself will require MONTHS of extensive testing and tweaking... and then secondarily, because we would rather isolate the content additions for 1.4.5 (and any needed fixes post launch) and then do the same for Crossplay as two bespoke things. Putting them out there together at the same time would be a maze of complexity that would frustrate the hotfixes for both 1.4.5 and Crossplay.
Most of these require a separate account signup (e.g. you need a Microsoft account to link all of your Minecraft's together... even if you are not playing on Xbox), or are server-side games (e.g. Fortnite). The real challenge is taking a non-server side game and enabling Crossplay without requiring some additional account for you to keep track of - and without the limitless funds and resources of a AAA company.
Not necessarily? As you can see from the WIP Menu, Crossplay is a toggle.
Again, to be VERY clear, the portable Altar is only available in very specific circumstances... this in not intended to be "place Altars all over your base".