Hi khampol, personally I think Microsoft funding PlayFab is a great news since the service is now free for starter games, and there is now a new premium plan that offers scaling, where they only charge you based on your amount of service usage.
Regarding hosting your own service, there is already a built-in Construct 3 object to do that, called AJAX, where you can connect to your own server, this is all manual though since addons determining the design of servers doesn't make sense, therefore any addon is not necessary for this! Just adding more work, with no benefits. Also, since you are already writing your server-side, might as well use the scripting feature in Construct 3, to save time.
You can find documentations of these implementations on the web, like NodeJS.
However, recreating PlayFab features in your own server is incredibly difficult and time consuming, would usually require you a dedicated team to handling that, which is ironically significantly more expensive and time consuming than using any service. Although, if you can narrow down the features, it is possible in barebones.
I don't think we would be making something like that any time soon. However, we do plan to add more game services like PlayFab, in the collection, in the future.