Treasury Secretary Andrew Mellon would’ve been a better successor to Coolidge, although ultimately I wish Coolidge could’ve served one last term.
I don't think Mellon was ever interested in the presidency. By 1928, he was rather elderly, especially for that era when life expectancy was much lower than it is today. Mellon did a good job as the long-time Treasury secretary, spanning the terms of Harding, Coolidge, and (partly) Hoover. He can be considered as the father of what later became "supply side economics."
As it turned out, BTW, Coolidge died of natural causes three months before Hoover's term was completed, and very concerned about the future of the country under President-elect Franklin D. Roosevelt.