Considering the political situation at the time, I don't think Aral or Cordelia could have given Gregor that choice. Cordelia certainly would have wanted to, but I think even she would have realized it wouldn't be possible. Gregor had to be Emperor. There were no other alternatives that wouldn't lead to civil war. And, even now, if Gregor dies before there are more children, there may be no choice for the new Crown Prince.