I like the combo of Nimmo and Alvarez, although if Atlanta were willing to offer up Wright or a prospect of that caliber, I'd much rather go that route. Nimmo is plenty intriguing for a variety of reasons but he's certainly a commitment in the direction of a re-tooling, which in the end is still something I think has to take place. Like it or not, whatever window of contention we aim at will require a substantial number of pre-arb players, which we already have right now. That still could be the case if we aim for 2023 as the first year of major contention, but in that case some of the current pre-arb players will need to be moved in due course, while others will need to be re-signed.
Ultimately it comes down to the best value available. I don't think the rebuild is so clear cut given the roster situation, even setting aside what the other NL Central teams are doing (frankly, the Reds should improve and be pretty good, but they lost almost 90 games last year; the Cardinals may end up getting David Price in which case I'm sure he'll turn amazing overnight, etc). I don't say this to suggest that we can immediately compete in 2020, just that I don't see any of the teams as currently loaded with talent or having talent emerge in the near future. The Cubs have plenty of current question marks and are set to fall off a cliff after 2021.
Getting back to Nimmo, in a move to get him, another factor could be signing him to an extension. This kind of move would be a calculated risk, on both his potential and his health. He is currently not a better player than Marte, which is why he might be the odd man out in a trade. But he was also a dark horse MVP candidate for some people last year before he got hurt. Having Newman with a little bit of speed, and all of Newman, Reynolds, and Nimmo with good to great OBP and contact skills, plus at least a little bit of pop in 2/3 of them would make for a good foundation for the offense if everyone is healthy.
If it's Nimmo vs two 45 FV prospects with some question marks, I'm taking Nimmo, but that implies other things. Marte's value should probably still be more than that, but who knows where the actual offers are at right now. There's a lot of smoke with the Mets, and I don't think it hurts that other possible destinations seem to be Arizona, Cincy, Philly, or Atlanta, all of which are at least potential WC competitors for the Mets.
I'd want to run the values before fully committing to this, but an ideal package might be Nimmo, Lowrie, Petersen, and Alvarez. Especially if you risk an extension on Nimmo, then you line up years of control with the current core more, add a mid-rotation pitcher who is close to ready, and then get a valuable bounceback IF and a longterm prospect. Sure, Nimmo could continue to have health issues, but he could also immediately put up a better season than Marte. Lowrie contributes right away, potentially allowing you to collect a couple more assets if you flip him, and you can also add some assets by flipping Frazier immediately this winter. Then when it comes to Bell, either you make the investment and have him as part of the core, or see what kind of big offers you can get for him. Same goes to some extent for Archer and Kela, as guys likely to be moved during the season, especially if they bounce back (though Kela is a good candidate for an extension IMO, but I doubt they do it given all the bullpen drama).
All of this is just giant speculation based on one possible strategic course that can be charted if a package oriented around Nimmo is the best one. But if so, we should definitely be absorbing the entirety of Lowrie's salary in order to name the additional pieces from their ok-but-not-great minor league system, especially but not only because we'd be able to do that without even really altering the payroll for next year.