John Cazale, the actor who Fredo, died of cancer in 1978 and Fredo would be in his 80's by this time so...Not sure if anyone would like to see a guy that old try to get respect or revenge, especially since all those involved in his downfall are all dead now.
I agree, messing around with the original storyline wouldn't work, and besides, the killing of Fredo is in keeping with a 'Godfather' movie. It's what made Michael Michael.