Currently, all mountains contain the same 4 features - an underground river, a chasm, a lava lake and the abyss/daemon area. There's variation in how far each of these is from one another in each mountain, and of course the rock and minerals are different every time, but it's still all the same.
An idea I was thinking of would be to add in some other mountain features that could be randomly inserted into the mountain, spicing things up.
Some ideas I've had:
A cavern - just a natural cavern, that could be anywhere from very small to very large.
An anciant hall - I'm thinking "Halls of Moria" kinda thing - a big hall, with pillars down the sides.
A crypt - A room filled with coffins and/or sarcophagi ... and maybe skeletons/zombies, who are pissed that you've disturbed their slumber.
An old treasure chamber - filled with gold/gems/etc. And possibly a defender (golems? undead? dragon?)
Lair - A small settlement of ... something. Maybe frogmen in a pond, or a hive of antmen. Anyway, they're unhappy that you've come.
Anyway, that's just me thinking for half an hour or so, I'm sure there are tons of little things people could come up with.
Each mountain could contain a number of any element, or none. So maybe one game I run into a bunch of caverns while I'm tunneling, but never see any crypts or lairs. The next I hit a bunch of antmen hives, and have to deal with that.
You'd have to tweak the numbers and probabilites, I'm sure, but even just a few of these things would help make each mountain a more 'alive' and individual place.