Displays and optics are a big one, lots of competing technologies there like OLED vs LED, pancake vs fresnel, movable optics, laser based displays, video pass through vs semitransparent or even HUD style for lower res overlays. They can also compete on audio quality for microphones and headphones, different battery solutions like hot swappable packs, corded or built in. Maybe different form factors that can distribute weight or higher quality head straps for comfort. Tracking options like more cameras for inside out or a different system for pairing with controllers or full body trackers. Even external dedicated compute that works with Air Link. If they’re making the hardware they can add whatever extra chips or sensors they want.

That's not even considering the possibilities of using eye tracking for foveated rendering to combine high resolution and long battery life

> Qualcomm don't make a higher end VR chipset

I don't think that's a given long term. Even already the chip in the Quest 3 is rated at the same GPU grunt as an nVidia 1060, the minimum supported chipset originally for PCVR. The next gen is already announced and is significantly more powerful again, able to power 4k displays. I would project in 3 years from now we have something that can actually be considered at least moderately high end in stand alone form factor from Qualcomm.