If you're designing a new computer platform, do you care about power (i.e. does it need to run on batteries)? If so you probably want a RISC architecture rather than CISC (although the Intel Atom tries hard to be a low-power CISC machine). That leaves you with two choices: Arm (used in all smartphones, some Chromebooks, and Raspberry Pi) or RISC-V. Arm is more mature and there's a lot more software available, but it's not an open source architecture.