I´m not really sure if interoperability of blockchains is a requirement for this.
There are already wallets like Jaxx that support a lot of different cryptocurrencies:
https://jaxx.io/features.html
It sounds like a good point to me but not for storing multiple currencies, instead just having 1 crypto, say Bitcoin, that I could always use to send to someone else only willing to get paid in alts.
I could stick to using Jaxx or Coinomi or Exodus to store multiple crypto, but I'd still have to manage individual coins and private keys or worry about finding another "home" if the wallet stops supporting one of my coins (Like how Exodus dropped Dogecoin mid last year). Not to mention that I don't actually have the same confidence in those wallets - none of the ones I know are open source.
If true interoperability exists... then I could presumably just only own Bitcoin and still access alt-only services. If that's how it works, great!