We are lucky to have sentencing by jury in Texas, something which many states do not have. But, any sentence given by a jury to a convict cannot be greater than that allowed by law. Juries can only rule on a case put before them. Before a jury hears a case, a law has to be on the books, a law enforcement officer has to make an arrest, a prosecutor has to decide to bring charges, and a grand jury has to rule that sufficient evidence exists to proceed with a indictment. Juries do not get to charge a defendant with a crime different than that brought before them.
Juries can do that today.
But, judges have the power to overturn a jury conviction by mandating a retrial, reducing charges, or even acquiting if they think a jury has convicted without sufficient evidence. In fact that is how the Fully Informed Jury bill proposed changing the law in 1997. It is hard to see how information from a defense attorney to jurors about their power to vote their conscience if they think an injustice is being done, will lead a jury to convict without evidence or not to follow rules of procedure designed to protect the defendant's rights. The main way our nation became one of laws, not men was to prohibit ex post fact laws in the Constitution. That way, no government official or ruler can arrest people and charge them with something no one knew was a crime when they did it.