After dismissing a few ideas that were interesting but not easy to work into a solid gameplay loop or produce during 48 hours (the topic of submarines came up a few times), I ended up going with a side-view mining game; think Dwarf Fortress crossed with Lego Rock Raiders crossed with Motherload. You direct a team of miners to venture underground and bring precious resources back to the surface, but as you go deeper it becomes harder to work efficiently - your miners need to head back up to eat or replace tools, and carrying rocks is slower than walking normally. So as you earn money you can build supply depots and elevators to assist your team.

https://www.youtube.com/watch?v=T8yoONdmWng

It's been a bit of a challenge to get my brain in programming mode today, but some of the functionality is coming together - the terrain automatically makes holes between open spaces, the player can set or clear tasks for the miners, and I have a basic pathfinding system in place (albeit there's still some bugs hiding in that one). First milestone on the to-do list is getting to the point that the miners can make new tunnels following the player's commands, second milestone is having those tunnels drop rocks to be carried back to the surface and sold (at which point we have a basic gameplay loop) and the third milestone is being able to purchase additional structures for the mine; we'll see which of those I manage to achieve tomorrow.