I agree on Patricia Cornwell (James Patterson is a similar example) but not on the Kathy Reichs part. Or at least, not for the first four of her books which I read and then vowed not to read any more because the plots were so tedious and uninvolving. The trouble is, I cannot know whether the later books are better unless I break my vow.
I can't think of examples of series that predictably get better with time. Some I always enjoy, eg Michael Connelly (Harry Bosch), Robert Crais (Elvis Cole), Arnaldur Indridason (Erlendur), Helene Tursten (Irene Huss), Henning Mankell (Kurt Wallender). Others vary a bit in quality, not necessarily predictably, eg Harlan Coben (Myron Bolitar), Lee Child (Jack Reacher), Sue Grafton (Kinsey Millhone) – but are usually pretty good.