I am not an EE, and so I'm running this conjecture on breaking out the GPU from the CPU by folks who have better knowledge than me. Perhaps you can point out something I don't know about modern CPU architecture.
Many modern processors provide integrated graphics, with the exception of some AMD processors which provide additional cores in some models (Ryzen 5 2600) and integrated graphics in others (Ryzen 5 2400G). Off-chip graphics currently use a PCI-Express bus, while on-chip graphics appear to the OS as some kind of IO device on some kind of bus (details unimportant: figuring out bus and device is what your OS does).
Per my understanding, a CPU may execute extended instructions such as for FPU; whereas a graphics chipset is a self-contained computing platform performing I/O on a working set on its own. Software does not intermix GPU instructions with CPU instructions, and the GPU doesn't operate on the same RAM; rather the GPU has its own processing frequency, its own instruction stream, and its own RAM space.
This suggests to me that a GPU off CPU would be sensible. Just as modern PCI-Express graphics adapters use a generic interface, a GPU chip could use a standardized socket interface. The CPU could connect it to a bus and provide RAM; while the output pins could route to whatever chipset encodes to HDMI, DVI, or other video output port.
A CPU could assign the GPU's video range to a DMA-mapped area of RAM, or it could allocate a separate RAM channel (e.g. one or two RAM slots) to a second MMU which services the GPU, thus avoiding contention with the CPU. An MMU capable of shared access can also disable its own access to a RAM slot and enable a secondary MMU, making this configurable. The CPU would allow the GPU DMA access to system RAM so it could copy data into its own memory space just like a graphics card.
This seems to leave one trade-off: the VRAM would be regular system RAM, rather than GDDR5 or LPDDR4 or whatever the manufacturer wants to stick on their card.
If the above is correct, then there's no real advantage to integrating the GPU with the CPU on non-embedded systems; whereas providing a GPU socket would allow pairing better CPUs with separate low-cost GPUs. A desktop PC doesn't make sense as a SoC, while a dedicated socket for a GPU makes sense because every PC needs some kind of graphics and the interface is routinely standardized across PCI-Express.
Integrating the MMU and north/south bridge makes plenty of sense due to the specific hardware interdependence and large amounts of data passed between each continuously: this saves latency.
Likewise, graphics card DMA would go across the MMU in the CPU, tightly-coupling these. The MMU would expose a chunk of RAM just like it works with integrated graphics; if the GPU wants to manage virtual memory, it can treat that as physical RAM and integrate its own MMU, thus avoiding the overhead of an off-chip MMU. This seems no different than on-die GPU.
Does all of this make sense, or is there some enormous advantage I'm missing (besides separate package cost) having the GPU on the same die instead of plugged into a nearby socket?