We released a series of updates to Cloud Save, including Queries, non-player data, Access Classes, Files and a new (easier to use) SDK surface this year - and we increased the storage limits significantly.
https://docs.unity.com/ugs/en-us/manual/cloud-save/manual/concepts/queries
Using Cloud Save with Cloud Code (and Cloud Code Triggers) should be great for this sort of use case.
Queries works with Cloud Code (as is in both the C# and JavaScript SDKs). It's also supported in an experimental release of the SDK for Unity, which I can share with anyone interested; it will be out in early January - it's code complete, just delayed release 2-3 weeks because of the holidays.
This is slightly different from the use cases addressed by Persistant Worlds, although there is overlap and we have been collaborating with the team working on that this year. As well has benefiting from integration with the Unity ecosystem, Cloud Save can be more cost effective than paying for a hosting for a dedicated relational database and solves for a huge range of use cases - everything from Clans/Guilds, to shared Quests/Community Goals, Auction Houses, card games, Battle Passes, player rewards, etc.
We recognise there are some use cases call for more advanced features though; for situations like those you can use an external database with Cloud Code - with Cloud Code Modules (C#) you can use any database, for Cloud Code Script we've also enabled MongoDB support out of the box.
As some of these features are new - many of them went live just head of Unite - we are still working on building up examples for these use cases that show how to build them, but the features are live and we are already seeing folks starting to use them. If you have any questions would encourage folks to use the forums as I know a lot of staff engineers and engineers who work on those features are active on there!