Right-handed pitcher Sonny Gray
Truly one of the most underrated pitchers in the game over the last two seasons, Sonny Gray would be the most expensive (and therefore the least likely) option on this list. He's been nothing short of incredible in his two seasons in Minnesota, and will most likely get Cy Young votes this year.
He's posted ace-level numbers with a 2.84 ERA, 1.15 WHIP, and his 2.84 FIP tells you that it's not a fluke. He doesn't allow home runs and he gives his team a chance to win every five days. He and Pablo Lopez will anchor a playoff rotation for a Minnesota team that could be a dark horse in the American League.
Obviously, this one is probably not going to happen, but I included it because he still shouldn't break the bank entirely as he isn't young. Next year will be his 12th season in the big leagues and he will be 34 years old. Once again, the Pirates severely lack veterans on their pitching staff, which is why he couldn't be a better fit. Skenes, Gray, Keller would instantly be one of the best rotation frontlines in the National League.
All in all, the Pirates should prioritize quality over quantity this upcoming free agency. A few quality players would go a much longer was than a bunch of the usual bargain bin players that fans are used to. The team has shown what they can do this month despite having two starting pitchers, so it's not hard to imagine what they could possibly do with five.