Matt-
When you say "There are no constraints for it to look like the existing templates, this should be a completely new design." ... how far are you willing to go with that? Each template now depends on a specific set of Smarty variables passed on a per-page basis.
So you couldn't put, say, ticket details and invoice details on the same screen because they come out of two different PHP files ( clientarea.php?action=invoices and supporttickets.php ). If designers come up with designs that meld data like that, are you willing to make the needed modifications, or does it need to fit within the existing confines of the Client Area pages?
D
P.S. Brett, fully agree with the idea of using something standards based, e.g. Bootstrap!.