The Psion needs to be a separate class that uses the Warlock spellcasting chassis.
The Warlock is a popular class, with unusual casting mechanics. It so happens, its mechanics work well for psionic flavor, such as always on cantrips, invocations to augment effects, thematic spell selection, slot-9 spells, and so on. On the one hand, the Psion can benefit from the Warlocks success, such as feats and items that improve the spellcasting chassis. On the other hand, the Warlock can benefit from having an other class that uses its mechanics to gain more future support.
Flavorwise, the two classes have little in common. One is about innate power of ones own mind, the other is about the power of some external entity. But the mechanics are great. Probably, the Warlock chassis is unusual enough for the Psion to feel like a Not-Wizard.