In the opening scene of the novel at Connie's wedding, Don C. is talking to Tom about "The Senator", who the Don himself advised not to attend, but who sent a gift of antique silver, "very valuable", that, as hagen points out, "The kids can sell for a grand at least."

Puzo then writes

Don Corleone did not hide his pleasure that so great a man as the Senator had shown him such respect. The Senator, like Luca Brasi, was one of the great stones in the Don's power structure.

Given the fact that he was placed on a par with Luca Brasi, and that a Senator (A Senator)) could certainly be more valuable to the family than a simple muscle man, (no matter how unique Luca Brasi may have been), don't you find it somewhat disappointing that no further mention was ever made of him?

If I were reading the book for the very first time, I would have certainly expected the character of the Senator to have been at least partially developed.