I honestly feel like this will end completely opposite of this, steven will be the only one to die in the whole show, it fits his personality perfectly to have that be the ending, he'll kill the diamonds freeing gem-kind from their tyranny and in the process someone will kill him and the crystal gems will arrive in time to hear him say goodbye and the last few minutes will show how gem-kind and humanity travel the stars in peace making the universe a better place, and that is what I belive will happen