Currently there is support for specifying an item_id when creating a content item in ArcGIS Enterprise (e.g., the add method for ContentManager in the ArcGIS API for Python.) Please add the same support for ArcGIS Online.
Our primary use case for this is to restore items that were backed-up and removed from ArcGIS Online, so that references to the item are not broken by the item being assigned a new, different item_id when it is restored.
Many organizations have developed our own backup scripts for ArcGIS Online and there are several products that do this as well (see the ArcGIS Online Back-ups - hosted feature layers and web maps/apps thread.) While useful for a number of backup scenarios, none of these solutions are able restore an item that was deleted at the original item_id, so references to the item are broken. That means a restored web map, referenced by a storymap, ends up with a different item_id, therefore, the storymap no longer works, until someone edits the storymap to reference the restored web map at its new item_id.
Enabling one to specify a specific item_id when creating a content item in ArcGIS Online would make it possible for such solutions to restore an item without the need to track down and repair broken links to the item.
As part of the backup process, one might capture the original item_id as a tag on the item as it is backed-up (similar to how the clone_items method adds the source item_id as a tag to cloned items.) Then you have the original item_id to use in the restore process.
