The RPi4 SoC already has PCIe support; it's used for the USB controller, but it wouldn't be impossible to have a version that has an M.2 PCIe slot. It could be used for NVMe, WWAN or faster Ethernet.

USB could remain available through USB OTG and it would sit in the middle between the RPi4 and the Compute Module 4. Using the USB power socket for OTG is doable already on the Pi4, but to do it in a fully supported way would need some extra components to properly handle USB-C configuration. Still cheaper than an H2plus