So this isn't necessarily about just PS5 Slim, but I thought about some specs for a PS5 Pro (IF there's one coming) and think these could make some solid specs for it (while also being reasonable).
CPU: Same as PS5's, but higher clocked and on 5nm. Maybe something like 4.2 GHz would do well. Would still be Zen 2-based.
GPU: Either RDNA 3 or RDNA 4, but keeping the same number of shader cores (2,304). The TF increase would come with dual-issue shaders like RDNA 3 is doing, and can still keep the same clock (2.23 GHz), for a little over 20 TF.
I dunno what RDNA 4 will bring design-wise (in fact no one knows outside AMD), but maybe they will also decouple the backend clock like they did with the frontend logic? If so I can see the backend logic (ROPs, TMUs) running faster, say 2.6 to maybe even 2.8 GHz, but otherwise ROP & TMU count stays the same. That would bring pixel fillrate to 179.2 Gpixels/sec and texture fillrate to 403.2 Gtexels/sec.
They may also increase the frontend logic clock, or just stay with the same, since they can do 12 primitives per clock with RDNA 3 (compared to 8 with RDNA 2). So primitive rate could increase to 26.76 Gpolys/sec to even as high as say 33.6 Gpolys/sec, triangle rasterization at 13.38 Gpolys/sec - 16.8 Gpolys/sec. Honestly though, I'd expect the frontend logic to stay at 2.23 GHz.
RAM: The base PS5 technically can support up to 512 GB/s of bandwidth with the way the memory controllers are designed; if we're talking about a design that can actually service as a base PS5 spec replacement too (though this may probably require it on 6nm instead of 5nm for costs reasons), Sony can tweak the memory controllers to support higher-bandwidth GDDR6 modules for a Pro version and keep the same 14 Gbps ones for the base model (which would functionally disable the dual-issue shader mode and implement single-issue shader mode on the GPU). So the base would have the same 448 GB/s bandwidth, but a Pro model could take some higher-speed GDDR6 modules (let's say 20 Gbps ones) to get 640 GB/s bandwidth...
...or they could just save that budget for MCD options. Remove the cache on the MCD controllers for base PS5 models, keep some for the PS5 Pro models. I don't know how much cache would be too much before the cost benefit is lost, but I think 32 MB could be doable, and maybe they can go for slightly slower modules of RAM (18 Gbps) to balance that out for the Pro model, so 576 GB/s bandwidth.
SSD: Doesn't honestly need any bump-up in spec, but they might redesign the form factor for the internal drive for both base & Pro to use regular m.2 drive format and keep some private NAND on the motherboard to store encrypted OS files (including an image backup), and user OS config settings. That way they can reduce the complexity of the motherboard & system design and still let users upgrade the internal drive.
I can see capacity increasing to 1 TB though, at least for the Pro model. Base model? Might also get increased, or maybe they reduce it to 512 GB to drive value proposition to the Pro model. Even so with the compression PS5 has on file sizes that 512 GB would get a lot of mileage.