It is now expected that the universal DH rule will be implemented in the 2020 season and beyond. This rule will help the Mets more than pretty much any other team in the MLB.
The Mets in 2019 had ridiculous hitting depth coming off their bench. Jed Lowrie, Dominic Smith, and JD Davis were the main pieces. In 2020, this depth will be even better, now that Yoenis Cespedes is finally healthy enough to play again. Having these players wasting away on the bench while a lineup slot is taken up by a pitcher was hard to watch last season. This season, we can have Cespedes play DH if he is not healthy enough to play in the outfield, and we can give him frequent days off if need be and plug in Dominic Smith, JD Davis, or even Pete Alonso (and play Dom at first). Last season, Dom Smith hit for a .282 batting average with a .881 OPS, filling in as a bench bat and occasional starter when rookie phenom Pete Alonso needed a day off. JD Davis batted .307 with a .895 OPS when he finally got consistent play time later in the season. Without a pitcher taking up a lineup spot, the Mets lineup could look like this: Nimmo, McNeil, Conforto, Alonso, Ramos, Davis, Cespedes, Cano, Rosario. This is a lineup where arguably the worst hitter is Robinson Cano. This would be the 2nd best hitting lineup in the NL, trailing only the superteam Dodgers with Mookie Betts.
The addition of the universal DH coincides pretty much perfectly with the likely return of Yoenis Cespedes from his ankle surgeries. Cespedes can slide straight into the DH role, and will thus take a lot of wear and tear off his surgically repaired ankles since he does not have to track down balls in the outfield. Cespedes is in a contract year, so his performance in the shortened season will be extremely important for his future in the MLB. If he can perform well in the DH role for the Mets in 2020, that will be extremely beneficial for both him and the team, because it gives us an extra threat in the lineup while not having to take away JD/Dom in LF. Cespedes is a notoriously streaky hitter, and in a 50 game season, this can be hugely important. If Cespedes can go on a hot streak or two, he could end up carrying the Mets to the postseason again (although the hitters in this lineup are much better than in 2015).
The Mets in 2019 had too many very good hitters coming off the bench because they did not fit into the lineup anywhere. Adding the DH in the NL allows them to alternate these players into the lineup based on performance when last year they would only be getting occasional starts when a starter needed a day off, and these sporadic starts did not allow them to gain any consistency. With the addition of a DH spot into an already formidable lineup, the Mets hitters become even more of a threat, and one of the best lineups in the NL.