How about a physical form for Cortana. The cartographer could have deposited her a machine or the UNSC could stumble across her while fiddling with the machine. Then hook her up with some bad ass humanoid robotic suit.
How about playable Cortana as Chiefs back up in halo 5. Thinking EDI style (ME3).
U never know something could have gone haywire with the cartographer and she has been put in a human body on earth.