The designer, Mike Fitzgerald, just mentioned in an interview that he has signed / sold to Stronghold Games the follow-up game to Diamonds.
The next game uses the same "suit action system" as Diamonds. The follow-up game introduces a wild card, a "faux diamond" which "sells the lead diamond and moves the track". Mike mentioned that it will have a stock market element with multiple boards.
It will be released in 2019.
Mike Fitzgerald's interview with The Dice Tower's Jason Levine starts at the 7:58 mark.