Infiniminer. It gets neglected a lot for being a minecraft ripoff, but it actually pre dates minecraft, and minecraft originally started out as an infiniminer clone.
Link: http://thesiteformerlyknownas.zachtronicsindustries.com/infiniminer/
I haven't looked at the code recently but the game that inspired Minecraft is open source (Infiniminer).
Also to compete in ludum dare you have to put your code up so there is code for thousands of games on that site.
That would be awesome. The original installer is on an archive of the developers website
http://thesiteformerlyknownas.zachtronicsindustries.com/infiniminer/ Didn’t know there was open source, thanks
Slay the Spire's the only one that feels like it was designed as a combo from the start.
Minecraft was heavily influenced by Infiniminer, in turn by Infinifrag.
http://danthemanhathaway.com/ProgProjects/BlockEngine/infinifrag2.html
http://thesiteformerlyknownas.zachtronicsindustries.com/infiniminer/
http://thesiteformerlyknownas.zachtronicsindustries.com/infinifrag-version-1-1/
Infinifrag seems like, they were influenced by Team Fortress but added the ability to modify the environment with blocks. So if you're going to say "FPS + Lego" combination, you're really talking about Infinifrag, and Infiniminer and Minecraft are really about reducing the emphasis on the FPS side to put more emphasis on building.
But then Fortnite brought back the emphasis on the shooter side. Original Fortnite was survival mode, which I've seen described as Left 4 Dead + Minecraft, if you want to think of it as genre combos.