Nomad Camp is basically a Woodcutter with a penalty; you don't tend to want it to be gained onto your deck. Woodcutter is already not a very good Woodcutter.
Nomads, now there's a sweet Woodcutter. So often there is some cute trick it can do.
Duchess had to be weak, since you could take one for free with a Duchy. It was still too weak. Man, it could at least have only looked at your own top card, without letting your opponents do it too.
Guard Dog has no connection, and is another new Reaction. Okay so. If they play say Witch, you can react to play your Guard Dog. If they play say Militia, don't react, just save Guard Dog for your turn, and then you'll draw 2 extra cards. Of course there are also all the other ways you can get the 4 cards. It's another one that plays into the filtering theme.
I like Cache, except for it being bad. If only I could move the +Buy from Margrave onto Cache (and from Wharf onto Merchant Ship). But I can't. And I mean. It's bad.
Souk is unrelated, beyond being another when-gain card. It rewards a small hand size, such as you might have after playing some filtering cards. And it gets in another when-gain trigger.