ARM (thumb 2, Cortex M0-M4) is by far the most important target. Second is cores like Cortex A7 and A53. I think AVR is pretty minor outside hobbyist circles.

ARM (thumb 2, Cortex M0-M4) is by far the most important target. Second is cores like Cortex A7 and A53. I think AVR is pretty minor outside hobbyist circles. After ARM support, I'd rather first see MSP430 (amazing for low power), some FPGA softcores (like NIOS, microblaze, etc., common in low volume, industrial & medical devices) and even 8051 (these buggers seem to still be everywhere, but I guess targeting Rust for this arch is the ultimate challenge.). I guess RISC-V would be cool as well, I can see this getting more popular in the future, eating ARM market share.