Yeah interoperability and the ability to travel between worlds is key. Just the other day I implemented a portal between two virtual worlds in WebGL, it works on mobile/desktop but as stated in this thread the experience is ruined when in VR because selecting a link pops you out to a 2D view. To sustain the feeling of immersion, the user's state needs to be maintained when hopping between worlds.
This is something the folks at Oculus Browser/Meta are considering though! Ideally it needs to be a standard, just like HTTP and TCP/IP. I'm incredibly bullish that the metaverse needs to be built on top of our current World Wide Web, so the ecosystem itself isn't owned by any one corporation. The destinations themselves, sure, they can be properties of a startup or larger co like Meta or Microsoft.