I wouldn't be too sure that FFC did not at least suspect there would be a sequel. Two reasons:
1) When Michael tells Fredo threateningly in Las Vegas after Fredo defends Mo Greene: "You're my older brother and I love you,but don't ever takes sides against the Family again. Ever." FFC seems to be setting us up for the day when Fredo does again takes sides against the Family, and Michael makes good on his threat, in GF2.
2) When Michael settles the Family business at the end of GF, you would think that Fabrizio would be at the top of the hit list; and indeed, FFC did film a scene, never used, where Mike himself kills Fabrizio. But there is no hint of Fabrizio being targeted in GF, perhaps because FFC knew he would hit him in GF2.
Harlan Lebo, in his authoritative "The Godfather Legacy," says that Paramount committed to GFII several months before GF was released. But at the time of the commitment, all the principal photography for GF had been completed except for the Sicilian sojourn, and some minor second-unit stuff.
