I would love to see an m.2 connector on the bottom of the raspberry Pi to support perhaps smallish 30mm or 42mm M.2 NVMe storage. We don't need huge capacities, so we don't need the longer 60mm, 80mm and 110mm sizes. Yes it would make allot of cases obsolete.... as they'd need to have taller standoffs..... Another option would be to put it on the edge of the Pi. Either way, it would be really nice to see 64GB or 128GB of M.2 NVMe PCIexpress storage on a future Raspberry Pi 4B+ variant w/ 8GB of ram. Probably need to wait for the Rasperry Pi 5...
The M.2 slot is the main thing. Performance-wise, whether SATA or NVMe isn't actually going to make much difference, for a machine like the Pi.
The biggest argument in favor of NVMe is actually the simplicity, since the SoC already has PCIe, which is all you need for NVMe. If they wanted M.2 SATA, then they'd also have to fit a SATA controller, somewhere on the Pi's PCB.
The second argument for M.2 NVMe is that it's also more versatile, since there are other PCIe boards you could put in a M.2 slot.