I think the more customizable the better, personally being able change the ceiling level could make the game feel bigger or more claustrophobic. Changing enemy sizes would be cool to experiment with too. Plus I didn't know the arms had actuators until now.
I would love to have just a pure skeleton and to allow people to add armor and stuff on top of it. Or change some parameters to have a different look for various parts (hands!) or at least have some variations. I don't know how much possible it would be and it is not top priority. Enemy size could be fun too and I will be investigating that.
The actuators in the arms are only to make arms longer. There are also rails on which the arm moves. Arm without any armor plates looks weird but you see everything inside moving (stretching, twisting, rotating). ATM there are lots of things hardcoded but I always start with a hardcoded stuff for meshes and then I replace magic numbers with parameters.