I would absolutely love to see a middle ground between "not touching your subject at all" and fully-pleasuring them. Like, maybe you could take one finger and lightly twirl it around their navel, or nipples. Or maybe twirl your tongue around the tip of their dick. Something to make subjects squirm, giggle, and fidget around that isn't overstimulation or "regular" pleasure.

Inserting a tiny camera into urehtra / vagina. Let it travel up to and beyond the cervix or deep inside the bladder.

It would make for a nice piece of equpiment to have a monitor that needs to be purchased for such tools

Test subjects could start to tremble with growing anticipation and upon seeing preferred actions/tools about to be used on them.

E.g. when they are into sounding and a tool is about to be inserted into them.

One of the things I was disappointed with in the first Rack was the lack of titfucking. I mean where the subject would be straddled.

like cock growth or muscle growth already suggested. test subject grow larger overall

Maybe some of the test subjects require you to use a condom... Or maybe sometimes cumming in condoms is just plain HOT. These things could be either crafted or ordered in bulk.

Sheathes, Balls, etc. Please?