I would really like that to happen. As long as it's not some reoccurring character and instead they introduce them to a new pony that has cancer or something. Though I would put my money down on it that it would happen. MLP is a bit more juvenile compared to Steven Universe and Gravity Falls and other shows like that, and the show is supposed to make you happy, not cry. Well, I mean, you can cry at certain scenes that are intended to be sad I guess but I personally haven't found anything in the show that has caused me to break down. Or shed a tear.
Also, this show is made to sell toys, and I don't think a cancer pony wouldn't really boost sales up.