It is not for everyone and it's not even ready yet. But the new ARM powered RISC OS A9 looks rather like it might be one possible shape of the future.
The desktop PC is very much ready for an evolution into something smaller, more secure, etc, etc. The Mac Mini and several other machines show us that. The A9 is far different from what we have now, but it is fast, efficient and miserly in terms of power consumption. I almost want one.