I can predict what the next RaspberryPi will be like, not because I am psychic, but because of what the RaspberryPi is: a small, low powered developer and hardware hacking board, meant for applications as diverse as an electronic doorbell display to a wireless print server.

Much of that functionality will be lost if it ever becomes much more powerful than it is. It needs to remain USB bus (or even PoE) powered, so it will always be under 5 watts. It will never gain PCI-e slots or the hardware to use them, or support for SATA, or expandable memory, or external GPUs, or any of the other basic things that desktops really should have. And it needs to remain cheap, because nobody wants to build their own microwave display with a 200 euro ARM board.

At best, it will simply get slightly faster, with a slightly more capable GPU (probably focused towards hardware-accelerated 4k video), get more onboard RAM, and have slightly more throughput on its SD card/USB/network mixed interface. It will never be anything more than a novelty (and very slow) desktop if used in that function, because, and I stress, it simply isn't meant to be.