Currently if you are using a secondary server calls across multiple sites and want to gain a de-duped number in a global report suite we need to deploy primary and secondary calls using a single third party cookie. It's becoming increasingly common for third party cookies to get blocked by some browsers and mobile devices.
To combat this problem it would be useful if we could have a blended approach e.g the primary server call uses first party cookies, then secondary server call uses third party cookies. We could take this one step further and on the back end processessing consolidate cookies where possible to preference the first party cookie.
Alternatively Adobe should also consider purchasing or developing a finger printing approach to unique user identification to help combat this issue.