I doubt Bungie would give us that, but it's nice to fantasize. Instead of Challenges, I would design it so that every third wave features a random modifier, i.e; Wave Three implements Juggler; watch your ammo, Wave Six implements Angry, and so on. Every third round past nine would incorporate two modifiers, i.e; Wave Twelve implements Juggler and Angry, Fifteen implements Lightswitch and Arc Burn, watch those Dreg. As you progress, each new set of "Waves" adds a modifier, until you hit a wave that features every single modifier. At this point, if your Fireteam hasn't already been wiped out, the game would give you a random Exotic and the Guardians in the Tower would erect a statue to immortalize your accomplishment.