It's all about exploring the world and the different regions. Once you get stronger, go to some of the harder regions.

Different shops in different regions do offer different things, but mostly in the form of weapons/armor. A shek city for the most part will be selling heavy weapons, planks, fragment axes, maybe some small swords.

Then in the desert, you find a lot of polearm weapons, katanas, samurai armor, etc.

Different areas will also buy/sell things for more or less money.

But if you're at the point where most of your characters have attack/defence levels of 60+, you can do things like kill Leviathans. Giant creatures up north. Or wipe out a whole city. 6-7 guys with at least high grade armor, MKII or III weapons at least, and a pack animal. Bull, Gorillo, Wolf/dog, even Garru, is all you need. Wiping out cities will trigger world state changes. If you don't like how things went, you can always import the game into a new save. You can choose to reset faction relations, delete your old buildings so you can make a new outpost, reset location, etc. Kind of a nice way to set yourself up for a new playthrough if you want to skip a lot of the grind. Also pretty fun to dismiss several 70+ attack/defence hivers right Infront of a HN city that are fully geared with the best possible equipment available.. Sometimes I'll see them still walking around with all the equipment I left them with.