Based on the planned changes for 6.4 (ZBXNEXT-8087) to remove template nesting I would suggest to allow to define Macros that can be used in Hosts inside Host Groups.

One use case where we use template nesting a lot is in multi-tenant environments. We are using nested templates to define/override top level template for tenant specific macros, example, SNMP community names.
For example: For example a generic Vendor Device model template, let's name it Ven_Dev_SNMP that we use as a base. Then we generate Tenant1_Ven_Dev_SNMP, Tenant2_Ven_Dev_SNMP ... etc. In each of these templates we usually had some Macros overridden. Most common case - SNMP communities, API users, sometimes also some thresholds and timers.

This simplified new device onboarding and configuration a lot. As we do not have to specify this information in a per-host basis. Just adding the device to a correct template.

If template nesting will be removed - we will need to invest more effort for configuring devices or a very good tool to manage the "cloned" templates.

For such use-cases I would suggest to allow to define Macros at a host group level. This could allow us to get basically the same functionality and from my viewpoint this would be even a more suitable place to define such Tenant specific Macros.