Extract: 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

FWIW, I still continue to believe as far as GFIII goes, that Vincent sort of came out of nowhere. I don't know if it would have improved the movie, but it would have been more believeable to have one of Sonny's boys going into the life. It just seemed like, although not impossible, a stretch for the audience to buy. But, that's my opinion.