Lately I haven't really had any interest in improving Digger, but I still occasionally think it might be fun to write a sequel. "Digger 2" or "Digger's Adventure" would have a 2D continuously scrolling playfield, impenetrable walls, new monsters, bosses, new items to collect, more weapons, shields, warps, speed-boosters, and maybe the possibility to dig to the surface where it would be possible to jump.
You know for the longest time (a year or more after I started playing back in 1986-7) I didn't know you had the ability to shoot. I played through the games first 5 or 6 levels before someone told me that digger could shoot.
Yeah it took me a while to figure that out too - it's not very discoverable!