With HBM they could go for larger iGPU's, but I don't think it's worth it. GPU intensive like 1080p gaming is best and most economically addressed by discrete graphics. (Unless you're a multibilion multinational like MS or Sony, in which case you might have the economies of scale to make a 7nm semi-custom APU with HBM worth it versus the dGPU route.)
For PC's, the APU's are great at value in the mainstream performance range. They now handle 720p (and somewhat above) really well. For anything at or over the performance of RX-560 one cannot beat discrete though.
Now I hope 7nm APU's come in two forms. MCM and a mobile oriented monolithic, with the MCM version being as capable (or slightly more) as the 3400g in the graphics department, while doubling the ability in the CPU department.
I could see the HBM stacking for the consoles. They are going with chiplet too most likely to improve yield and get the costs economical. It's possible the CPU chiplet would be identical/reused between Sony and MS. The memory controller may sit on the CPU chiplet (maybe?), and the GPU might be on one or two chiplets (2 more likely?).
But on AM4 they'd have problems packaging that. They might be able to fit a small GPU with a single stack of HBM in the spot of one of the CPU chiplets (but that GPU would be so small that I don't think it'd be worth it, and the costs for doing that wouldn't be), and I'm not sure they can move the I/O module's placement to put HBM by it. There's just not much room without changing the whole packaging and I'm not sure they're ready for that yet. So it might not be feasible until they can stack onto the HBM.
I think they both already said they're using GDDR6 on the next consoles so that's not gonna happen. The memory controller would either be in the I/O or on the GPU chiplets, as they'd have to have a special version of the CPU chiplet made since it has no memory controller at all (and definitely not a GDDR6 one).
Sounds like the rumors are suggesting the MCM APUs might have been cancelled this round (I think Anandtech mentioned that in the interview with Lisa Su, and she said they hadn't said what that product would be but that its not cancelled - which I have a hunch just means there'll be monolithic Zen 2 APUs), and I'd guess that's due to the packaging issues of AM4.
AFAIK the CPU cores and GPU cores are custom variants due to having some secret sauce built-in for backward compatibility so not much interchangeability between the 2, they should be able to use the same I/O die thou.
They say that its custom but I personally am skeptical that it will be drastically so. I'm very doubtful the CPUs will be, but the GPUs might.
Oh and I really don't think it would be for backwards compatibility since that shouldn't be that difficult since its x86 CPU and the GPU still adheres to general API. It'll need tweaking to run on the new hardware for sure though (Microsoft even said they have frozen adding games to backwards compatibility so they can focus on having what they've currently got working on the next system).
Also with regards to that general talk, the rumors I saw said pretty much the opposite, that the dGPU Navi we're getting this year is a stopgap between GCN and the full architecture changes that is the basis for Navi, and that the next gen consoles will have the "real Navi" (and I'm guessing Arcteryx will bring most of that to the PC GPUs).