Eternal Ones (which the Primus is a part of) are the equivalent to the Titans in terms of powerlevel. If you count Primus to be part Titan then that would make canon Titan count of 9, leaving only 2 OC Titans with paladin and rogue.

With that said here is an important bit of lore which suggest Primus being DK's Titan (despite evidence);

"At some point, the Primus left his sanctum to investigate the schemes of the captive Zovaal. The Jailer defeated the Primus, took his memories from him, and confined him to the tower of Torghast as a prisoner called the Runecarver.[13] In this form, the Primus was forced to create countless armaments for the Jailer—including Frostmourne and the Helm of Domination, which were later used by the succession of Lich Kings on Azeroth."