When I first ran Steading of the Hill Giant it was a major surprise for a few of the players.

Wait! Purple skinned elves? Who live underground? Are you kidding?!

What would be a surprise today?

Something about the D&D world that makes players sit back and go "whaaaa?" Some subversion of expectations.

For clarification: you're not asking for a story hook to surprise your players; what you're looking for is a piece of mechanics from the game (new race, class, magic, etc) to surprise them?

If so, it could be difficult. I really liked the idea of Dunamancy from the Critical Role books, which is essentially a new school of magic that controls gravity/space and time to an extent. It was a neat spin that I thought fit very well within the game. 

one of my favorite forgotten mechanics is the fact that everyone's favoite insane fish people can alter reality.

thats right, these guys

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Pick a monster race. Have them be socially accepted, surprisng the players.  Goblins, kobolds, ogres, whatever.   Let the players get used to them as being 'good'.  Then have a whole plot about a secret plan about/against them. 

Make most of the campaign be "Are they good? Are they bad?"  Some of them clearly are evil.  Some are not. Throw in a Lawful Good Cleric/Paladin of the race just to make it clear not all are evil.  You could have the big bad guy be one of them attempting to cast a mass domination spell on the other members of his species, turning them into his evil mininons.  

 Best part of this is it makes for a great story if the Heroes FAIL and the species becomes evil.   Then you could have a whole nother adveenture about rescuing them or at least some of them.

"[X character] was a shapeshifter all along???"

 miniature cloud giants. a nearby observatory of mages and clerics is baffled upon catching a glimpse of falling stone below the clouds that they didn't observe above the clouds. curious. you've got quests for collecting a sample, quest for escorting a translator when it's discovered to have runes on it, quest to find other fallen pieces, quest to aide an arcanist in developing a spell/item that levitates this particular peculiar type of stone, etc. then discover through trial and error (and story nudges?) that the the spell/item reverses time on the stone to catapult it back up to whence it came! experimenting further, players might find that standing upon such a stone and then 'reversing' it would carry them to the clouds (among other applications). now explore that. what's up there that even the aarakocra don't know about? do they require breathing assistance? is it haunted? is the place all abandoned ruins or was the first fallen stone just a fluke?

basically, i'm saying steal a really fun mechanism from the recent Zelda: Tears of the Kingdom nintendo game. steal it so hard! players could go on to discover and reactivate ancient fantasy robots, but if you've stolen enough from zelda already then maybe... maybe, miniature cloud giants? like a whole society of goliath/elf/dwarf/genasi/whatever humanoids living in the clouds and as surprised to see you as you are to see them. do they tend the clouds like sheep or keep them as boats? maybe they do fortune telling based on the strange constellations they see on the ground at night (city lights, on ship lanterns at sea, etc). do the big cloud giants know of them? do they have an arrangement with dragons? are they all the elaborate plot of a elder oblex who was long ago stranded and now isn't sure how to react?