I like both of your ideas.
Since it's about moles, all the characters need the ability to dig through walls, and that ought to be a major factor in the game. Adjust class balance by changing the speed with which they can dig, and maybe the amount of waste dirt produced. Also needed: the ability to use waste dirt to make new walls.
Yeah, I want to have exactly that, lots of digging mechanics that are sort of like what you see in Dwarf Fortress a little, though more combat and escape oriented and less architecture oriented. Stuff like causing cave ins to bury persuing monsters, openning up pitfalls under unforunate monsters' feet. I think all moles will be able to dig in some limited capacity, but only miners will have the combat digging moves availiable to them or they will be the only ones that usually reach a high enough level in digging to access them. Mole Muggers will be able to dig sneaky things if they have enough time though, like foot holes and pit-traps and the like, eventually being able to make dreaded spike pits.
I like both of your Muncher ideas. I'm leaning more towards something like what Bongo said though just because that way it can be more of a general game mechanic that Munchers excel at than yet an other Muncher only mechanic (like his bite attacks). I think that is more intune with the general feel of roguelikes in that, all classes can attempt anything, some just excel in certain areas and some are comepletely worthless in some areas. Also I want to have items like armour and weapons, and a muncher eating a weapon and then gaining a bonus, while playing to his goofy persona as a bottomless eating machine, is going a little to far into the absurd maybe. Him being able to eat a lava worm and not only beable to stomach it but actually benefit from it where a Miner attempting the same thing would just burn himself terribly, I think works just as well as a sort of goofy ability that a expert eater would have and yet retains some logic.
Just for further reference for the other classes
Miner - Heavy armor user and weapon master who is good at digging (since high weapon skill equates to good digging skills)
Mugger - Crafty rogue, master of speed, stealth, disguise, unlocking, and valueable item apraisal.
Mystic - Wise soothsayer, master of magic, alchemy, and all other things arcane such as magical item identification and enchanting