Time to judge.
Yeoman by alion8me: Nice and simple, and interesting to reward diversity of card types. There will be some boards where that diversity just isn't available, but on the right board it could be a lot of fun, especially if you have some control of what you draw for your next hand.
Cull by anordinaryman: I like how the sifting, drawing and trash-on-gain help each other, if you go for multiple Culls. The more diverse your deck, the more likely you are to draw 3 or 4.
Bridleway by Aquila: A cost reducer that doesn't encourage megaturn strategies. It rewards diversity in terms of what you buy each turn, and you'd want to be buying at least three differently-named cards in order for it to feel worth it.
Weird Sisters by chronostrike: This looks almost strictly better than Witch; you'd be unlucky not to be putting at least two cards in your hand, and discarding duplicates often works in your favour too.
Secret Society by D782802859: I like the concept, and it seems very strong, being able to set up your next turn at night. It's great for engines with at least three components (I'll top-deck my village, my terminal drawer and something else) but it doesn't offer any incentive to get more diverse than that.
Caste by Fragasnap: This encourages diversity in terms of card cost, which is a novel take on the challenge. It's nice to play your terminal drawer first and then hope to have some villages or cantrips to choose from. I'm struggling to get excited about diversity in terms of card cost, however.
Vulcan by gambit05: It is good how this rewards getting these diverse cards into play, not just into your deck. I wonder if it might be a bit too strong for 4, but nice and simple and I like that.
Student by grep: Interesting to reward diversity of card types and, being a Night-Duration itself, this ensures there will be at least 5 types available on the board. Little bit of sifting gives you some control of what you draw for your next hand. But so many words for the student to study on this card puts me off a bit.
Exploiter/Exploited by Jonatan Djurachkovitch: Strong enough that you really want one of these, so it's going to impact any game when it's on the board. I like how diversity is encouraged, to reduce the impact of the attack. I think the non-stacking of the attack and the self-weakening of the draw ensure it is not overpowered.
Goonies by LibraryAdventurer: Nice to see a less powerful, friendly, Goons variant. The reward for buying cards that you haven't played that turn is interesting but I think in a not-so-fun way. You either buy cards you won't want to play, or avoid playing cards that you do want to. And you'll get the +1VP if you buy a Province anyway.
Bribe by LordBaphomet: I like the concept, and nice to have a simple way to bring Prizes into the game. But as I discussed in #7215 and #7216 I don't think this event incentivises diversity.
Cornucopia by majiponi: It is good how this rewards getting these diverse cards into play, not just into your deck. Interesting comparisons with both Horn of Plenty and Harvest. It offers more predictability than Harvest, which is great. (I almost considered naming the challenge "Improve the Harvest".) The +3 Actions is helpful, but it would make me a bit sad if this was the only non-terminal in my starting hand.
Sundial by mandioca15: Introduces a mini-game around if/when to buy a less useful card just for the VP. Looks like it adds a straightforward first-player advantage. Also need to consider how this interacts with Gathering types; do you need to keep track of whether the Sundial effect has triggered for those piles?
Diverse Village by NoMoreFun: A village where you get to pick which card you draw, and you're more likely to find the right card if you've diversified. Potentially lots of deck-cycling too. Nice simple concept.
Tera by silverspawn: I like how Cities power up, and I like this, where you have to put lots of effort into playing diverse cards and then bam you've got second level Cities that only cost 2 each. Introducing another type of tokens just for this card puts me off a little.
Wartime by spheremonk: I should think this does change your priorities, in a diversifying way. I wonder if at some point the Silver and Gold become most attractive to avoid the embargo tokens (though sometimes buying Silver and Gold might count as diversifying

). I am put off by the prospect of so many tokens sitting on the card piles.
Yeoman by spineflu: +1 Action +4 Cards if you can get sufficient diversity/luck/deck control. In many games I think this will end up being a very swingy card. (Curiously another entry called Yeoman - are Yeomen known for their diversity?!)
Gossipmonger by Xen3k: I think I like this, with the sifting helping subsequent Gossipmongers find more juicy targets. There is a lot to resolve on each play though - could it benefit from costing 5, with slightly stronger rewards?
Hard to choose (I guess it always is), but I think my favourites are:
For the diversity impact it would have on boards which include it, I'm going to choose Exploiter as the winning entry.
Winner: Exploiter by Jonatan DjurachkovitchRunners up: Cull by anordinaryman and Tera by silverspawn.