How can we improve WHMCS?

At the moment, support for Single Signon in Plesk is via the mod_pleskaccounts, where the UserID is linked via the panelexternalid to the subscription on Plesk. This assumes that a user has all their subscriptions under one Customer login on a Plesk server.
As a result, if there are multiple different subscriptions, with separate Customer accounts in Plesk, Single Signon will take you to the first Subscription found, no matter which one you come from in WHMCS.

mod_pleskaccounts should be modified to include a field for the subscription ID , and have a different panelexternalid for each subscription, so that it logs you into the correct one.