Since core sets don't follow a specific world or story, they are, as has been stated many times before, a good place for universal fantasy tropes to get their due in top-down designs. There are some stellar examples of cards like this since Magic 2010, but still many more left that haven't gotten their due. For example:
A Dragonslaying Knight. Knights vs. Dragons isn't just the subject of a Duel Decks product, it's been a mainstay of fantasy since St. George. Rashida Scalebane does exist - but a mechanically similar and nonlegendary creature (like a smaller Baneslayer Angel) would be a good thing for white's Limited sideboards.
A Dwarven Smith. Core sets have plenty of one-shot creature types (even for sentient tribes) so there's no commitment needed to put one of the all-time favorite fantasy tropes (the Ultimate Blacksmith forging apocalyptic weapons) in as a fun Johnny/Timmy rare using the only appropriate creature type.
A Cool Ship. Yes, ships have historically been hard to execute right in Magic. But one core set card could be a test run for the shortlisted "Ocean World" block.
A Dark Lord. Magic - partly to its benefit and partly to its detriment - doesn't have any hammy pure-evil cackling villains. But Liliana would embrace that kind of role to her own ends in-character, and a planeswalker card showing her embracing that facade would be fascinating.
A Game of Chance. Well, one has to while away the hours in a seedy stereotypical fantasy tavern somehow. And none of the mini-game-type cards really represent a "regular" card game, like poker or blackjack. This would be a chance to glimpse a slightly offbeat world, such as a Wild West-themed one.