I've been reading this thread with interest. I'm a Houdini dabbler - at a very light level.
What really interests me is not trying to reproduce C4D techniques - eg: mograph setups, but learning what H can deliver than C4D can't.
Personally I like to learn the fundamentals, so I can take those and use them in whatever way I want. I find that much more useful and rewarding than projects that simply show 'how to make X' - but don't address the fundamental questions behind the process. If I understand the fundamentals I can then 'make Y'.
My interests are probably not common ones: Terrain geometry was the first thing I wanted from Houdini - it's got a great terrain system. Fluids - liquid and gaseous - and open water simulation is another area I'd like to explore.
Of all the training I've seen for Houdini I was most impressed with that at Hipflask.how I haven't invested in the 'geometry essentials' package, but having done the free taster course i am tempted.