Now THIS was a fun and original game! Been missing these fast-paced arcade style games!
Honestly I can only think of more characters. Someone rattled off a good list, but a few they missed are kobolds, sergals, lamias, lizards, fish people, etc. Which brings me to my suggestion...
The water is an interesting feature. It would be nice to upgrade to maybe a yacht and be able to encounter things on the water too. I almost wonder if a Volcano would add some more interesting chars and stuff too. One thing I don't see enough game devs do, is place filters on what fetishes are seen too. It's probably difficult and time consuming to do, especially if the chars are already made, but it's really refreshing to see filters for content you aren't into (gay, lesbian, futa, straight). I feel like this opens up the game to a broader range of audiences, but hey, it's just my opinion, and I'm not the one troubleshooting code. You do what you think is best for your baby! That said, I do have some very vague ideas that you could incorporate (if you want), as you move forward.
I do have some ideas, but I absolutely love the game as is. I don't think these would harmonize too much or should be mainstays, but it could be interesting to see these in minor facets, as long as it doesn't change the core gameplay too much. You can take or leave these as you please: Maybe a minor quest feature, some goals to reach perhaps? And a merchant feature that allows you to get certain benefits from mining, perhaps from a city or group? Or perhaps specific locations such as a meteor crater, swamps, mountains, town, or fissure, some with perhaps more rare types of materials? Or even rare events, that could shake up the story a little bit (like what if the drill broke at a certain location, or someone sabotaged the vehicle. Maybe the vehicle accidentally hit someone and now you gotta room them. Or perhaps a char could get too fat and the vehicle would break down, etc.). It's gonna be cool to see what you do with the characters interacting with each other (at least I think that's what you're aiming for) it'll be cool to see what interactions there might be!
P.S. If you're up for a Round 2, I would also love to see a sequel to this, an Underwater or Sci-fi Space theme comes to mind. There's probably a LOT to explore there! Can't wait to see what you do, love what is there thus far!