Sure thing No Robert Duvall / Tom Hagen No brainer

My take as well, there were too many new “lame” characters who popped up in Godfather 3 that didn't fit in, instead of cultivating the "existing previous characters"
eg: Sonny and Sandra's son/s, Michael's son and even Connie's son/s as power struggle schemers instead of inventing the new character Vincent, Sonny and Lucy Mancini's son, out of nowhere, fast-tracked to Don Corleone

Andy Garcia still could have played Santino Jr. or Frank

I feel in Godfather 2 the new characters Pentangeli and Roth, in spite of the absence of Clemenza [granted not the big vacuum as no Hagen in Godfather 3] and Clemenza's mysterious, unexplained death - somehow pulled it off

Both Pentangeli and Roth's connection to the Corleones were believable!