It would be incredibly useful to allow Organization Admins to set the default thumbnails for newly created items in AGOL. There are already default thumbnails that vary by item type, like the default web map thumbnail attached below, but it would be very useful for an organization to change what these default thumbnails will be for items created in their organization.
Our organization tries it's best to follow a particular style with item thumbnails, but sometimes the number of items we create out-paces our ability, time, and frankly willingness (specifically for internal and intermediate/temporary items) to create a new thumbnail for all of these items.
Being able to set/change the default thumbnail for web maps, service definitions, feature layers, etc., would help our organization look less like a jumbled collection of stray items within AGOL and would provide a convenient customization option for organizations.
I imagine this idea as the option to change the default thumbnails by item type as the organization's admin.
