The first problem I see is that this is a hard conversation to even fathom until we have a better idea how Joe holds up over the course of this season. I'm going to make an assumption that the Joe we've seen thus far will be pretty similar to the guy that plays the whole year. No, I don't think he will hit .400 (though that will cement the HOF discussion if he does), but I could see a vintage Joe season of a .320 BA, .400 OBP, and a high .400s SLG. That won't be a 6 WAR season at 1st, but it would be pretty respectable, and in an MLB world where there's way too many 2 outcome type guys, a guy like Joe that can get on base more than anyone and has plenty of doubles power to go with it with GG defense. If he does significantly worse, then I'd probably pass, but as long as that OPS stays over .800, I don't see a scenario where upgrading Mauer in the short term will be easy or cheap.
In the minors, you have Vargas (who I'm guessing will not return next year as a 6th year FA) and Rooker. I like Rooker, but I think it's a stretch to say he's going to be ready next year at this time. Realistically, he's likely going to be ready at some point in 2019, but most likely not on April 1. And that is probably a best case scenario. Rooker could stall in AA for all of 2018 and still be a decent prospect, but that would push is ETA to 2020 if that happened.
In the majors we have LoMo for 2019 and an opening at DH if Mauer is gone. In 2020, LoMo is gone. Rooker will likely be in the conversation, as could a guy like Lewin Diaz. Perhaps one of the really young guys could surprise, but I'm not sure I'd count on that personally. Bottom line is that other than Rooker, I don't see anyone coming up the pipe that could be a plausible option for 2 spots in 2020 and 1 spot in 2019.
That brings me back to Mauer. If he continues to hit, I think there's a good fit here. As a matter of fact, a great one. I don't think a 3-4 year deal would be smart, especially at 36. I do think a 2 year deal with a club or vesting option for a 3rd makes a ton of sense for both parties, especially if Mauer wants to stay. I think he gets somewhere in the 6-10M range per year, and that will depend a lot on how the season plays out. If it's vintage Joe, then probably closer to 10... and I'm fine with that. What I don't see is the team going crazy in FA and getting the best bat out there. We all want that, don't get me wrong, but this will also be a that time where they are doing long term deals with the current core. That high priced deal might mean a guy like Rosario or Kepler ends up being shipped away because they are too expensive. I think Mauer provides plenty of value, and I also think that it's a bit underappreciated in the two outcome baseball that we see today.
So in short, if he keeps hitting, I'm giving him a 2-3 year deal.