Totally agree, this simple addition would make creating new documents much easier. Similar as setting pre-definted text styles and margines for in-design documents.

Yes, copy and paste between documentsworks, but given that XD doesn't support pages within the one document this is cumbersome and leaves more room for error as people do forget these things.

If you copy an Artboard from a file and paste it in another file, the custom grid settings come with it. You can then click "Make Default" to update defaults for the new doc.

The ability to make a custom grid a default choice within xd. To have the ability to use that custom "default" grid on it on any new project you make without having to rebuild a custom grid for every new project.

Ability to choose a custom grid and set it to your "default" program-wide.

For instance, making your own custom grid for bootstrap within xd and being able to set it as your default would be very useful to anyone trying to make a bootstrap build their end goal for their prototype website.