Customise Tenant invitation Emails

We need the ability to customise the tenant invitation emails. Some scenarios:

1. Inviting external users for a specific purpose they might not be expecting (Bootcamp invites, Nintex Internal use-case).

2. Inviting business users to a second test tenant for participating in UAT

3. Onboarding citizen developers - linking to internal training, guidelines, coe mateirals, helpdesk etc