JUDGEMENT
Honestly I'm glad there weren't too many entries, judging a winner was tricky; a lot of good entries:
jakav's Field/Farm/Town
Field, Farm and Town is a cool, understated application of the mechanic. Fields are skippable Coppers, but by colliding two with $3 other cash we can get through Farm (a light discard attack) to Town, once per turn, for some nice gains. It takes some effort to set field up as payload, but then roughly 4 fields = $4, 2 gains, a mild discard attack, 1 VP, and a village per turn for 2 stopcards; so the cost potentially ends up justified in the mid/late game. This isn't really what I was expecting when I set this contest, and defining Farm as an attack because it 'sells people food' is a bit of a stretch, but I appreciate this is a nice cohesive set of cards.
spineflu's Savings/Panic
Great flavour: we're dipping into our savings until they're gone, and then panicking - just like in real life. Balance-wise, I would say that Savings feels overpriced as it is; +2 coffers total on a duration for $5 feels simply worse than Villain. Even if we put in the work to uncover Panic, its discard attack doesn't seem very effective, and stacks poorly. With some rebalancing, I think this combo could be a lot of fun to play.
Erick648's Warlord/Enforcer
Great self-synergy, good flavour. Enforcer handsize attacks our opponents, so Warlord can gain nice cards from our opponents' hands. Furthermore, Enforcer lets us "exile" cards we've discarded to handsize attacks, or set up attacks to use with our extra warlord plays; albeit Enforcer can often suffer from the harbinger problem if our opponents don't buy into Warlords. Warlord's effectiveness also depends a lot on the kingdom; Supplies for example is a hard counter to the reveal gain, and horse/spirit gainers in general are problematic, though there's little to be done about it from a design point of view.
exfret's Sniper/Cover
In this instance, Cover is a hireling-esque action, that gives a one-time pseudotrash option, and a novel continuing attack to sift away our opponents' good cards. Cover is probably worth opening for the pseudotrash alone; but as exfret points out, the repeating sniper attack could get tedious. I think perhaps the non-permanent duration implementation of Cover had better interaction/flavour with Sniper, and there's definitely a lot of potential here.
mxdata's Demagogue/Knave/Embezzler/Dunce
This is a cute mechanic; Demagogue sets asides actions from the trash to play next turn; the Outcast "zombies" for this trash themselves at cleanup, preventing a player from monopolising any given Outcast. My main concern is that Knave is very mean; not dissimilar to the War hex, but far less random to trigger. Embezzler on the other hand will typically be a very weak option for the payload of a $5 cost duration, and maybe less so than Dunce, can vary in effectiveness depending on what you end up drawing for your next hand. Knave can set up other <$4 options to gain; a lot of effort for playing a terminal duration $5, but it could be necessary to replenish villages in the wake of the attacks.
mathdude's Preparation/Burning Arrows/Trebuchet/Smoke/Trojan Horse
These cards look like a lot of fun to play with. I enjoy the flavour of having an arsenal of siege options, and the interactions between the Plan attacks create neat combos to justify acquiring multiple Preparations. Preparation does feel underpriced at $5 mind; it isn't far off Captain, where Smoke alone is close to being a Ghost Ship, and perhaps isn't even the strongest option while curses remain to be trebucheted. Maybe call it $6, and consider renaming Preparation (could be a military equivalent of "Captain"?), and this looks great.
WINNER:
mathdude's Preparation/Burning Arrows/Trebuchet/Smoke/Trojan Horse
Chosen because it just looks like a lot of fun to play with. Congrats mathdude, and thanks everyone for your submissions!