Ooh. I don't know too much about a secret ending like that. If you need to unlock all the other endings to get it then I don't think ANYONE would really play it. It takes a long time even if you're speedrunning a single year and if you're doing that for multiple endings you'll have to grind for a lot of hours just to play as a mutant. It'd take hours to code the mutant route only for like 1% of players to actually see it.