Sure. Keep the DIMM sockets and add HBM to the CPU package.
Actually probably the best possible architecture. You can choose to have both or only one, backward compatible and future proof.
Yes, it adds another level to the memory hierarchy but that can be fine tuned.
You are also overestimating how much room there is on the interposer.
As someone with a 9950x3d with direct die cooling setup I can tell you there is no room.
Sure. Keep the DIMM sockets and add HBM to the CPU package.
Actually probably the best possible architecture. You can choose to have both or only one, backward compatible and future proof.
Yes, it adds another level to the memory hierarchy but that can be fine tuned.