The ability to create our own placeholders would be super useful.
You have already for years had the ability to put placeholders from tickets, service items etc so this is an obvious function that is needed and used by many if not all people using workflows.
However, many times there is hardcoded details in workflows that I have written in that need to appear in multiple places in the the workflow or in other workflows. Think if you send an email via a workflow that has a contact detail/email in it, if that email needs changing you have to go to all the workflows that it is in (and remember where it is..) or if using PowerShell app a particular application name might need to be used.
So if similar to credentials you have the ability to store placeholders in the admin section which are then available to use in workflows, you would be able to store an email, contact, department, name of any type to be used. if/when inevitably it changes you only need to change the placeholder and not each hardcoded workflow.
So some technology you already have (placeholders) tweaking slightly to give flexibility to admins cutting down this admin time so they can get on with more exciting things like AI.

