You mean a world boss? That idea is 15 years old and a good one hence, why would Bungie bother doing something good? Not to mention the 16 player limit in patrol instances would mean the boss would have to be pretty weak in order for it to be taken out. That leave it open to glitches, and cheating. Nope, unfortunately Bungie built their game based on Bad architecture and isn't scalable to have enough guardians in one area to take on a world boss. Just look at the Crota public mission. Way too easy.