This sort of questions will invoke random answers, you will get lots of very different personal opinions.
If it's only about the money then creating 3D art is probably not the business to be in at all. But if you're looking to get most money on 3D model/products creations then you definitely need to be in educational visualizations.
Things like internals of the human hearing system, the hard and lungs, internals of the eye, visualization of germs and viruses and the inner workings of the cells, etc.
Also molecular visualizations, atomic/quantum effects, plasma dynamics, nuclear energy reactors and fusion reactors like iter/tokamak and NIF, things like electron microscopy, particle accelerators like LHC, etc.
Also electronic engineering, things like internals of the CPU, quantum computers and its workings, etc.
Also in the field of energy and sustainable engineering, things like inner workings of wind turbines, solar arrays and other alternative energy supplies, etc.
Astronomy and deep space travel, for example curiosity rover, rosetta mission, kepler telescope and its inner workings, mars mission, and other anticipated missions, etc.
Basically things we would need mass education about.
If you can do things in this field, visually appealing and in great details, then those are rely valued items and people are willing to pay more for that sort of things.
Ps. "women" is the keyword used most often ;-)