Hinterlands has lots of little near-synergies, and disappointing synergies.
For instance:
* Haggler is good, but not as good as you might first think, with border village. It's good with highway, but not any better than a card that is cantrip money. To say haggler is good with +Buy is true, but not terribly exciting, since they are all terminals.
* Trader can improve a lot of cards interactions, but the payoff is silver? And if you use it on Ill-Gotten Gains, the silver doesn't even go to your hand
* Highway has surprisingly little synergy in the set, given how much of a combo enabler it is across all sets. the best remodels in the set are develop and farmland!
* Oracle improves Noble Brigand, but you don't really want either in your deck unless the kingdom is weak.
* Stables "combos" nicely with the shitty trashing in the set.
* Develop is nice with border village, because you can trash a 5$ into a 4$, 5$ and a 6$, but keep in mind that you lose the use of the 5$, and the 6$ is only a vanilla village in your deck.
* Jack of all Trades combos with the discard, but pretty weakly, since draw to 5 isn't inspiring, and you are probably discarding things you'd be happy to trash with the Jack.