I would travel back in time and make Acorn RISC OS go mainstream. It's a light OS with a full desktop environment, capable of playing video and rendering anti-aliased fonts, that boots all from ROM and runs on an 8MHz ARM CPU and 512 KB RAM.
I said "go back in time" because I'm not sure how advanced it is compared to modern systems, it's single-user only and has cooperative multitasking, instead of preemptive.