Limited travel to and from Boston/Providence. I'm still a n00b myself, so I still need to fast travel to Providence, but I'd like to propose a limited amount of free travels (let's say 20-50 times).
You will rightfully claim that this will disrupt trade, so here's what I also propose.
Buildable trading stalls.
Buildable Luigis & Clarksons. Not really buildable Luigis and Clarksons, but some building or object that has the same effect (a coach house maybe?).
Let me explain why I came up with this rather weird idea.
Like I've already said, I'm still a n00b and, like all n00bs, I like to explore. Recently, I had to go back to Providence to buy some nails (only to come to the conclusion that i didn't have enough silver, but that's an entirely different story) and I decided to walk. Apparently, I live rougly 1 to 1.5 hours from Providence and I had a lot of fun walking, initially.
Eventually, the trip became boring though. No roads, no towns to speak of, no trading hubs whatsoever, no perfect spots for thieves, burglars and highwaymen to rob me, etc. Sure, there were lots of new things for me at first, like spiders, frogs, snakes, a mischievous gnome, but IMHO, besides a few developed areas (developed is a big word.... 1 or 2 houses with a similar amount of leanto's and a random mine) I didn't manage to find any real civilized areas, until I got very close to Providence. Since there's no real need to develop areas (we can simply fast travel to Providence) and since we couldn't even if we wanted to (I checked and market stalls cannot be built, travel between existing towns has to take place either through Providence or by walking, in which case it's probably very easy to get lost), I don't find it strange that I didn't come across any.
How would one find a civilized area? by paved road of course. It's safe to assume that a road usually leads from somewhere to somewhere, so as soon as you stumble across a road, you can follow it either way and probably eventually end up somewhere (let's just hope it's not a robber's den, but then again, that'd be part of the game & fun). This would give new players the incentive to build closer to said civilized area/settlement and help the game grow in general.
I doubt my ideas will be accepted by the diehard Salem players, but maybe some people will like the ideas.