It would save so much time to be able to add to/edit the default artboard dimension options. I have never and probably will never need both Apple Watch artboard dimension options, but basic common print sizes A0-A6 (if not all the way to A10) should be easy default artboard options as they are commonly used by many designers, both when opening a file and when creating new artboards in already open files. This should be really basic & easy and would make me very happy, thank you!

Illustrator needs this urgently, photoshop has had a similar feature for a while, it's a shame illustrator doesn't have the same!

The whole point in presetting templates is to aid productivity, if you can't add your own presetting what is the point in even calling it such. Another reason for me to question why the **** am I paying for your software!

This is pretty easily do-able in the legacy new document dialog.

Add documents to this folder to make them available in the drop-down list:
C:\Users\YourUserName\AppData\Roaming\Adobe\Adobe Illustrator 26 Settings\en_US\x64\New Document Profiles

If you want to make your own templates (*.ait files) and have them available via the "Templates...", save them here:
C:\Program Files\Adobe\Adobe Illustrator 2022\Cool Extras\en_US\Templates

locations will be different on OSX, they may be in a hidden library folder somewhere. The directory structure should be very similar though.

update:

The new document profiles location on OSX *might* be here:

Hard Drive\Users:[user name]\Library:Application Support\Adobe:Adobe Illustrator 26\[language]\New Document Profiles

this is from a site posting instructions on how-to for CS5, with notes about updates for CC versions up to illustrator 2015 (version 20)

Add an option to the "Saved" tab on the "New Document Window" to add your own Template/Documents. i.e I want to add a blank template for A5 size and always have it handy when I create a New Document.

we need a way to store comon page sizes in the profiles, A4 and A3 are there but i also use A2, A5, A6 and various other standard sizes !
I ahve created a custom profile but it only stores a default paper size, and no options to add paper sizes... also the print paper size options are less than my new prifile!!!