Optional bosses have played a role in many of WoW's dungeons, and I believe add to the overall package. They give people something to continue to work towards, and are often the 'real final boss' of the place, similar to how many felt Nightbane was harder than Prince in Kara.
With all raid content, it gets farmed. It's the nature of an MMO that content will be run over and over. With this in mind, and the fact that rep grinds have been removed from specific dungeons, I think the best way to go about this is 'collecting X number of doodad Z, that has a low drop rate'. I want to punch myself in the face for typing that last sentence, because I play WoW, and know how frustrating that system is, so let's take it one step further and explore other options.
Let's say we make doodad Z to be a 'Black Dragon Figurine" (our unlockable boss is a Black Dragon). These have a low chance to drop off any trash mob in the place, and have a guaranteed drop rate off bosses. The dungoen already has 7 bosses in place, and each wing could concievably have one more boss added per wing, during the dungeon revision process. So let's go with 10.
We're guaranteed 10 figurines at the end of each week assuming a full clear. Is 100 items too much to ask for the extra boss to unlock? Probably. Maybe not. That's two and a half months of full clears, less with the inflated number that trash drops will give. So we make the items not bind on pickup. Suddenly, there's a market for Black Figurines. They're sellable on the AH, so once a guild has the key, they can begin to sell off spares. That speeds up the process even more, but only later in the time frame. It also allows PUGs to roll for the items to make a tidy profit after the run, similar to how shards from unneeded gear are rolled for on 5 mans.
But without a quest chain that anyone can complete, it begins to feel dangerous to give any one person the ability to open the event, for fear that they may leave the guild. How realistic is it to ask that these items be 'bind to guild', and that, when activated, the guild itself becomes attuned? Is that even feasible using WoW's binding engine? It would give people a sense of accomplishment for their guild, and give people a reason to stay with a guild that has gone through the unlocking process together.
This thought process begins going down an entire different tree... how can we make people feel like they're really a part of their guild, instead of "orc warrior #6b"? Warhammer has a really cool system where guild rewards were a more integrated part of the actual game, instead of some meta thing that takes place outside of the client.
Anyway, the point is that I think continuing to work towards another goal in a dungeon you already 'have on farm' gives people a light at the end of the tunnel that they can feel like they're still productively working towards, rather than just gearing out alts, or loading the guild bank with enchanting mats.
Whether this is an quest chain invloving killing various members of the dungeon itself, a single key that drops off the Baron, or collecting Dragon Figurines is up for discussion.
