Using iCloud storage for multiple photo libraries

iCloud is associated with a specific Apple ID and iCloud Photo Library only uses the system library on a Mac so there is no feature to do what you want - you can both use the same Apple ID (my wife and I do) and have one library shared between you - but to have seperate libraries you need separate Apple IDs and therefore separate iCloud accounts

And your question implies that you are thinking of iCloud Photo Library as an offsite storage location, it is not - it is not a storage solution or a backup solution, it is a syncing solution to keep multiple Photos libraries in sync

As to future changes no one here knows - until Apple announces new features neither you nor I have any access to them - you can request features here Product Feedback - Apple

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Is your wife using a different Apple ID? If she goes to Settings/Her Name/iCloud, signs in, and turns Photos syncing on, can she then log into iCloud.com/Photos using a computer browser and see her photos?

If you are using different Apple IDs, you won't see her photos.

See this article Request and make purchases with Ask to Buy - Apple Support