I was 'umming' and 'aahing' on joining due to time constraints... but your post made me change my mind and jump in head first. With a blindfold. And no helmet. Into an empty pool. So you have made a great accomplishment today, :D.
This is my first ever itch.io jam. I looked for code restrictions and found none, so as mentioned I'll jump in too to help try and spice it up a little. My usual reason for not making game ideas to fruition? Motivation and time spent doing other things or games already made. So this will try to pick up my dev vibe.
I plan on making a top-down, sci-fi city builder using HTML, CSS, JS and jQuery. Graphics will be basic. It'll involve countdowns for each building made, and each building will require power. Power will help the buildings slowly accumulate more civilians which generates taxes per second. Eventually you keep building and keep making power. Then you upgrade buildings to make them more efficient. All of this is aimed at earning enough via taxes to continue expanding your city via an expanding grid system that costs increasingly more to purchase.
What will spice it up is random events that will require you to make decisions, which influence whether your civilians will like you or not (this will slowly recover on its own), which impacts taxes. I suppose it needs a fail state, so it'll fail when your city's happiness reaches 0%, in which case they infiltrate your missile silos and blow up your house. Or something.
I tend to aim high... so let's see how far to completion it gets!