I see this asked quite often, so I thought it would be good to have a good place to link for some suggestions on what few things to buy to provide bigger deck-building capacity.
Here's what I think based on the archetypes you prefer playing. Feel free to post your own
Last Updated: Frontier Justice
Fourth Ring Hucksters: New Town, New Rules, 2nd Core Set.
The introduction of Paralysis Mark in NTNR gives a significant boost to the control capabilities of hucksters, and Forget at J allows you to move easily fill the same value as Hex Slingin' with something that you can discard to power it, as well as Mongwau or Micah, or simply power your home ability. If you can afford it, a second core set will provide extra copies of Blood Curse and Hex Slingin' which will be invaluable.
Sloane Hucksters: Double Dealin'.
Sloane loves playing with low difficulty hexes and Double Dealin provides this in spades with both Corporeal Twist and Fetch. Furthermore you get Jose Morales who has one of the highest huckster skills in shootouts, allowing you to splash some Soul Blasts as free Pistol Whips. Plus Espinosa is always a good addition to your Sloaners and Mario Crane will allow you to experiment with Super Mario builds as well.
Morgan Gadgets: Election Day Slaughter, Faith & Fear:
Gadgeteers get new toys almost every expansion, but you get the most bang for your buck if you focus on the 3rd saddlebag which will provide you with expendable and cheap gadget dudes, as well as another decent drop-in shooter, plus Election Day Slaughter can let yo run kill jobs, without sacrificing your pull chances. Faith & Fear will provide a weapon to make all your squishy mad scientists nasty studs, as well as 2 amazing new starting Mad Scientists and a dedicated home to boot.
Law Dogs Gadgets: Double Dealin', Election Day Slaughter
Law Dog gadgets don't need a lot to work, just some additional starting scientists which Drew Beauman and Mortimer Parsons will provide and your good ole faithful Holy Wheel Gun which can make your cheap low deputy studs into beasts. Plus you can splash the Quatermans into your decks if you can afford the chance to fail some pulls.
Law Dogs Miracles: Faith & Fear, Frontier Justice
Miracles don't even exist before these two expansions, and by getting both of them, you ensure that you have a good selection of them to build your deck. Plus you get a new outfit in the Arsenal that can enable you to turn your spells into aggro and a new grifter to expedite that.
Fourth Ring Abominations: Faith & Fear
While Fourth Ring has been mostly focused on hexes, with the introduction of Oddities of Nature, you can finally attempt a spell light, or even no-hex deck. Simply use your cheap Pagliaccios to disrupt their offense and protect your valuable dudes, while holding your town square to make up for your low influence. You probably will still need to play a few hexes, but they don't have to be your deck's focus.