We already have memory wafers glued to our CPU wafers in the form of L3 cache. It's lower latency, higher throughput, up to a few hundred MiB in bigger models and can potentially be used without external RAM sticks (but I've not heard of using that feature outside of BIOS firmware early boot -- that's probably the only change we'll see). Sometimes it's DRAM, sometimes it's SRAM, its size varies quite a bit.