Agreed. Somebody who was considered was Frmr Mass Governor John Volpe.
"Former" governor? Volpe was the incumbent governor in 1968. He was originally elected in 1960, but defeated in 1962. He regained the office in 1964, bucking that year's Democrat landslide. In 1966, he was re-elected - to a four-year term, as Massachusetts changed its law. He resigned in 1969 to become Secretary of Transportation in the Nixon administration.
Volpe would be an interesting alternative to Agnew. He would not have to resign for any reason that I know of, so he would succeed Nixon if Watergate played out as OTL.
He had no Congressional voting record, but IIRC he was regarded as a liberal.