Lots of speculation out there about the Gubernatorial race in 2 years, a lot can change by then, but here's how I see things currently:
(1) Setti Warren leaving the mayoral office, hiring John Walsh, seems likely to be running.
(2) Seth Moulton's been more vocal about issues in parts of the state other than his own district, leading to speculation, chances unclear
(3) Someone is trying to convince Kerry to run, very unlikely. Especially if Setti Warren runs.
Baker's chances at reelection seem extremely high. He hasn't made any major blunders, which is what is most important to keep the incumbency edge. The Globe has had some nonsense about him being Pro on 2 helping him in cities, but I think that is clearly repudiated by the results of the vote. That said, I don't think his positions on questions 2 and 4 will actually hurt him much, if at all. He didn't really stick his neck out much on either of them and instead spent the 2016 election grinding at his job trying to avoid questions about the Presidential race. Grinding at the job will help come 2018.
2018 is a long ways away (in terms of politics), but I don't think Baker has a chance of being defeated barring a major blunder in the coming months.
I've seen and heard a few "Baker's first REAL test..." pieces in the past few months, but they were more smoke than fire. He's still very popular.
His weakest trait may be that he doesn't really stick his neck out for anything
. He's very calculated and he takes extreme measures not to come out too strong on one side of most controversial topics. Really, Question 2 was the only one. He seems to be taking great care to do just enough to keep his conservative base happy while not ignoring things that are important to dems. He has no choice in crossing the aisle in MA, but he doesn't seem to have too much trouble doing it. Most Dems who work with him actually like him.
South Coast Rail is a good example of his approach. A republican Governor can't come out and just stamp a seal of approval on a basic 50 mile commuter rail project that has ballooned to over $2 Billion dollars and expect republicans to support him. At the same time, the South Coast has cried foul over not having rail for decades and one of the biggest conservative strongholds in Eastern MA is within that region, outside of the cities. He needs the support from the region. So Middleborough makes sense from a political standpoint because it can be presented as progress (it'll take less time to build) and- as presented- it's less expensive. Win/Win. Obviously the reality is that it doesn't make much sense and it probably won't be much less expensive, but it buys him time and that's the goal. He's not rocking the boat and he's appearing to play nicely with both sides.
He's also done a good job of backing some typically liberal causes. Especially Mental Illness and Opioid-related investment. He's made some pretty big commitments in Human Services fields that you don't typically see from a Republican.
For those reasons, I see it as a tall task for a Dem to beat him barring a scandal or major screw up. Moulton or Warren may try to paint him as a fence-sitter, or indecisive, but he's done enough that he should easily be able to overcome that. I like Setti Warren. He's popular and a lot of people have him pegged as Gov. material. But I don't think he can beat an incumbent who is so popular. Even a Republican.
Seth Moulton is very popular, but I think it would be a mistake for him to run for Governor. His selling point is the fact that he's a Dem who happens to be an ex-marine with combat experience. He needs more than that to play well against baker in Massachusetts. Honestly, I think his veteran status and experience would serve him better nationally. Maybe not for president in 2020, but possibly on the ticket as VP. Maybe a cabinet position. Dems had a terrible job winning veterans and military folks in 2016 (as they historically do). That's Moulton's strength and he doesn't need that in MA.