It would be useful to introduce two new dimensions:

1. Website ID (or measurable)
2. Website name (or measurable)

Then Custom Reports could leverage these new dimensions and allow creating custom reports on Roll-Ups broken down by Website id or website name.

For these dimensions I reckon we don't need to create new segments. Although having segments for website id/name could be possibly useful to segment a roll-up and filter out some sites, it hasn't been requested yet.

The dimensions should be available when used in the "Top level" of the report (ie. the first dimension added), and also when the dimension eg. "Website name" is added below, for example "Country > Product sold > Website name" should list, for any given country and product comparison, all of the website that sold this product.

Testing

To check this new feature works as expected, we should create a Custom Report on a Roll-up, where the custom report includes the Website name and/or Website ID. The generated custom report should let us drill down into any particular website.