Interesting, although if you strip it down the idea isnt so much as a new idea but rather making an old one better.
Seems as though the only adaption (possibly) is that you aim to not have a predetermined outcome well actually it seems as though you will just have two outcomes and then have it go from there.
MY concerns are this, How effective would this actually be? In your example that you gave I saw only one outcome, which would be the players succeeding -why? Because The players would strap themselves with the most armor they could, they would come in the largest groups they could and they would annihilate any mob armies you throw at them the players would always vie for the outcome which is to protect the elder and so this outcome would always be inevitable. And you can't just simple increase the difficulty or just buff up the event team staff because then players would complain and say that the event was unwinnable or that it was impossible or the boss was overpowered etc.
If you truly want both outcomes to be OPEN outcomes then you run the very difficult task of coming up with scenarios where both outcomes are just as likely. Scenarios designed based on how the player would think and then make challenges accordingly.
A challenging scenario or climax for one could be this,
Players are trapped in a room with no where to go, armies of mobs are swarming. On the wall are a series of levers (a puzzle of some sort) Players then have to solve the puzzle as mobs are fighting them. However the mobs never stop coming, their are no waves of mobs either; just a constant ever growing stream of them.
Pros: Survivability is based on overcoming an obstacle however there are no clues on how to do this. The task is difficult but not un-doable. The challenge in doing this is you need to make the threat real, throw enough mobs at them to the point where if they don't solve the puzzle they will die; but not too many mobs to the point where there is no time to solve the puzzle because everyone is busy dying.
-Also there is the added challenge which is posed by what you propose.
Do you propose an ongoing story with these events? or one off mini-encounters?
There are challenges to both,
On one hand an ongoing story makes those involved feel connected to it and they feel much accomplishment once it is resolved.
On the other those who haven't been with the story or missed an event would feel alienated and hesitant to invest the time to join in on the adventure.
One off mini encounters aren't bad either however they generally feel much cheaper and that sense of accomplishment is nowhere near to the other option. The way I feel about these is that mini-encounters should be handled in the same way Runescape handles random encounters.
I think it would be cool if some gnome wizard popped up while I was fishing then teleported me away to some island to overcome a challenge etc. -Then perhaps the rewards for these could be items otherwise unobtainable.