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:
- Wild animal suffering (probably not recognised as a moral problem by the majority of the population)
- Aging (many people probably ascribe it a neutral moral value, maybe because it is rightly regarded as a "natural part of life". Right consideration but it doesn't imply its moral value or how many resources we should devote to the problem)
- "Resurrection" or, in practice, right now, cryonics. (Probably neutral value/not even remotely in the radar of the general population, with many people possibly even ascribing it a negative moral value)
- Something related to subjective experience? (stuff related to subjective experience that people don't deem worthy to assign moral value to because "times are still too rough to notice them", or stuff related to subjective experience that we are missing out but could achieve today with the right interventions).
Cause areas that I think don't fit the definition above:
- Mental Health, since it is recognised as a moral problem by a large enough fraction of the population (but still probably not large enough?). Although it is still too neglected.
- X-risk. Recognised as a moral problem (who wants the apocalypse?) but too neglected for reasons probably not related to ethics.
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.