One great example is the pain gap / access abyss. Only coined around 2017, got some attention at EA Global London 2017 (?), then OPIS stepped up. I don't think the OPIS staff were doing a cause-neutral search for this (they were founded 2016) so much as it was independent convergence.
In my understanding "Cause X" is something we almost take for granted today, but that people in the future will see as a moral catastrophe (similarly as to how we see slavery today, versus how people in the past saw it). So it has a bit more nuance than just being a "new cause area that is competitive with the existing EA cause areas in terms of impact-per-dollar".
I think there are many candidates seeming to be overlooked by the majority of society. You could also argue that no one of these is a real Cause X due to the fact that they are still recognised as problems by a large number of people. But this could be just the baseline of "recognition"a neglected moral problem will start from in a very interconnected world like ours. Here what comes to my mind:
Cause areas that I think don't fit the definition above:
But who is working on finding Cause X? I believe you could argue that every organisation devoted to finding new potential cause areas is. You could probably argue that moral philosophers, or even just thoughtful people, have a chance of recognising it. I'm not sure if there is a project or organisation devoted specifically to this task, but judging from the other answers here, probably not.
What organizations do you have in mind?
Open Philanthropy, Give Well, Rethink Priorities probably qualify. To clarify: my phrase didn't mean "devoted exclusively to finding new potential cause areas".