People managing a large Zabbix instance and therefore having the user role Zabbix-Super-Admin may most of the time still be interested in a subset of host groups and related information only.

How about allowing a Zabbix-Super-Admin to toggle between being a virtual Zabbix-Admin and a Zabbix-Super-Admin?

As a virtual Zabbix-Admin permissions are granted as for usual users. Except for the toggle button to evaluate back to Zabbix-Super-Admin, of course.

I don't know, maybe a Boolean attribute optionally send alongside concerning API requests?

While this is different to ZBXNEXT-2018, both may have some things in common in terms of technical pre-requisites.