#1. Retirement From In-Ring Competition
Leukemia is something that Reigns has struggled with for more than a decade, which means that he is well aware of the enemy that he currently steps up against and whilst he's managed to get the better of it so many times, it's an unpredictable disease.
This means that whilst it's completely feasible that Reigns could be on Raw to announce that he's ready to return, there is also the chance that he's there to announce that he will be forced to walk away from the ring.
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Having the level of treatment that Reigns would then require wouldn't allow him to travel as much with WWE, which would leave him with no choice but to retire from the ring for now with the option of returning when his fortunes change.
Of course, it is hoped that the announcement is much happier than this one and that Reigns will be back on WWE TV full-time in the near future, but in the interests of balance, this is still something that could happen.