I personally view Cell as somewhat of a precurser to modern GPUs. GPUs nowadays are a lot more flexible than people give them credit for. Xeon Phi was mentioned as a multicore CPU, but it's pretty much identical to GPUs from AMD or Nvidia, except that it supports x86. Of course, "supports x86" is pretty misleading, since actually using x86 on phi is quite a slow path, somewhere around an order of magnitude slower than the "normal" codepath. It's basically equivalent to the pathological worst case for SIMD divergence.
Anyway, modern discrete GPUs can directly access CPU side memory (this goes through the page fault mechanism), perform memory allocations and deallocations, and schedule kernels and draw calls to be run, all through the shader/cuda/opencl/API-of-the-month cores. In theory, you could execute an entire OS entirely from within the GPU, using the CPU as a simple bridge to I/O, though I don't think there's much interest in actually doing this. A lot of work rewriting pretty much everything for a massively parallel environment, no public documentation for the low level inner workings, and why do you want the OS running on the GPU anyway?
Currently, APIs (with the exception of Cuda on Nvidia and the OpenCL/HIP/No-Windows-Port on AMD) are severely lacking. Vulkan and DirectX 12 are steps in the right direction, but they're both suffering from a bad case of lowest common denominator. They're also suffering from their HLSL/GLSL legacy, which is missing a lot of important features of C++ (as seen in Cuda on Nvidia or HCC on AMD). It's now possible to write your own compiler targeting Vulkan and DirectX 12 flavors of LLVM (DXIL and SPIR-V are both dialects of LLVM IR), and there are several projects doing just this (though Cuda is the big target with Vulkan being a "once the Cuda backend is mature" target), but until a major industry player gets on this, we're probably stuck with the old shading languages. Or Cuda.
I do a lot of GPU compute programming, so I do know the architecture about as well as it's possible from public documents.