I'm struggling a lot to get a good idea to work on so I decided to get some help from the community, what you think the unexpected mechanic would be like,sorry my English isn't that great.

You could make something that looks like it is one type of game but it's actually an entirely different kind of game. For example: one that looks like the perspective is top-down (like 2d zelda), but is actually a side-scroller with gravity (like mario).

I want to see a game including weaponized cake, but whatever. Do the unexpected! where people would expect a jump-scare, do something hilarious ( have you seen Death Trips?). Lock the final door in the game. Make the player's gun malfunction. Something like that. Hope this helps.

I just decided to take the theme as Random because that's unexpected right? Totally, don't question it

Kisk if you need some ideas both my submissions are in, you can look at some of my ideas in both submitted games, look at the code or whatever! Hopefully something pops out that sparks an idea, I didn't have many unexpected ideas but I was able to attempt a few that sorta works! But mainly randomizing works to a degree and that factor is part of both games somewhat as well. Hope your game turns out well and hope yours do very well too in this jam. I joined this jam to be productive and get back into coding again since I took a long break. Kisk I know your gonna do great think of really crazy ideas and work those in the game they just might be unexpected.