Big item missing from WHMCS, and been on the request list since 2007 (and probably longer)
Ability for Sub-Accounts to be linked to multiple billing accounts.
Case in point;
I'm an SEO guy. I have to make edits on multiple users websites. Sometimes there's an issue and I need to contact the host.
Well if I already have an account at the host, I can't add my email address again for another SUB account on a CLIENT's billing account.
The only option left (and the one currently used) is that we must log in as the client into their hosting company's billing/support system.
Would be so much better if we can use our own login to enter tickets on behalf of clients rather than having to log in as a client etc.