I don't think that there is any chance of this happening. Platforms want to keep their users on their own platform and not spread across multiple. If your friends are already on one platform chances are that you come to that platform as well.

But I agree that it would be pretty amazing to see happening.

I wouldn't hold your breath for that. They want players to attract their friends to their platforms. If we could play / talk to each other, there would be no reason for us to buy the games for their platform. I would really like it, but it's less profit for them.

Would be nice but sadly very unlikely. Some games are cross platform like Diablo 4 though.

I would love that, but the EU would have to mandate it first. Even then, the platform owners would be kicking and screaming the whole way.

And unlike hardware devices, it wouldn't be cost-prohibitive to keep them isolated outside the EU.

I think it depends on the platform. For example, Steam would never federate with Epic Games Store, because that would make Steam users more comfortable with buying games on EGS instead of Steam.

However I think this could change if platforms decide to start integrating more with Discord. For example, if Xbox and Discord decide to share friends lists, I think that would be beneficial for both companies. This might encourage Steam to integrate with Discord too. I could picture a snowball effect happening as more platforms start to integrate more closely with Discord, which could eventually lead to a future where all gaming platforms have shared friends lists. (I don't think it will be true federation though. I think the different platforms would only share friends lists through Discord, not directly with each other.)

There's a big issue I recall with cross platform gaming. This is specifically that most PC gamers have a mouse and keyboard, and most console gamers have a controller of some sort. For FPS, the PC gamers would own just because of the input methods - this happened back in the aughts. And I can imagine for some games the opposite would be true. And while consoles I guess could restrict the input for various games, PCs can't really.