Personally I want to work with RISC-V, because it has better documentation, easy peripheral access and simple emulator TinyEMU. I already have written disassembler for RISC-V (it work better that LLVM one and it is integrated with my executable format analyzer framework, sample output), small mini OS for experiments with RISC-V. It is currently able to output graphics, run threads with preemption (no SMP), handle interrupts, receive virtio-input events (tablet, keyboard), show stack traces.