Just imagine it, get 5 life time assassinations and you earn a ninja armor coating (navy blue and black.) Get 10 perfect games and get a helmet themed around master chief for the marc VII, so many options and it allows people something to grind for.
There would be tiers of it, easy to earn ones these are coatings or visors, medium to earn ones these are armors and charms, and hard to earn ones these are mythic effects.
All require you to get X amount of a mythic metal, each would have 3 tiers. And must be done in match maker (bot games count.)
example NINJA Master
first ninja: Ninja sticker
5 total Ninjas: blue stealth coating
15 total Ninjas: spare sensors hip pack (2 threat sensor bulbs as a hip attachment.)
25 total Ninjas: hidden body mythic kill effect (cloaks the dead body on kill.)