Hello,
Just wanted to suggest the possibility of more geophysical features such as:
Hydrothermal Vents: where water percolates up from an intersection of magma and aquifer, rich in minerals but boiling water and magma mixing produces steam and much fun.
Faulting: lines where the layers are shifted against each other, and occasionally shift further apart, earthquakes could lead to interesting consequences for poorly supported rooms as well as ones cutting it becoming split and shifted apart.
Pressurised magma: most magma is actually under overpressure in the earth, so connecting it up would be a great deal more difficult.
Varying Viscosity: some magma is more fluid than others, fluid magma tends to form a cap with tubes of flow radiating downhill, while thicker magma forms domes, which build up into large piles of thick cooled crust with a less so centre, which collapse in a very fun way when too high.
Dyking and sills: In the real world, magma more often than not travels as a dyke, meters in scale, cutting up from a magma chamber, forcing through layers and away from the source in an arch till it breaches the surface to erupt, one of these intersecting your fort would be interesting. Sills occur where a dyke terminates at a tougher layer, spreading out along it over x and y directions, but not through z.
Various stages of evolution for magma: perhaps a volcano, the magma chamber mostly cooled with clusters of geodes at the top and many frozen dykes radiating from it.
I am not sure if these are possible to implement, or would be of use, but may well be interesting or infuriating.