By “holistic design” I mean that the Transformer should be able to convert from primary to secondary mode without the necessity of any parts being attached or detached. (I am not accounting for “accessories” as such in this factor. Accessories might be an aesthetic necessity, but they aren’t necessary for transformation as such)
This might be a controversial point because it would be a counter against the design of many, many Generation One Transformers. For example, quite a few characters required attaching fists to complete the robot transformation. All of the seekers, Sunstreaker, Trailbreaker, and Optimus Prime all had attachable fists. Making matters worse, these removable limbs didn’t really have anywhere to go during vehicle mode (although you could just toss Optimus’s hands into his trailer). Sunstreaker’s toy at least made an attempt to incorporate a spring-loaded feature into the toy’s forearms, so you could launch missiles in place of fists. The most obvious flaw of this feature is the bane of children at play across the globe: lost pieces.
One careless transformation, a hand goes missing, and your very expensive Transformer just became a permanent casualty of war.
There are worse offenders, though. For instance, Ultra Magnus proved less a Transformer than a construction set. One had to keep track of two sets of fists as well as a head. Plus, Ultra Magnus’s chest plate was a removable scout craft. And, of course, any of the G1 combiners relied heavily on attachable limbs, armor, and heads.
For the most part, Hasbro has been much better at “holistic design.” I’m hard-pressed to name a single modern transformer that requires separate pieces to be attached to convert from one mode to the next.
Indeed, many modern Transformers go as far as to extend holistic design to the accessories. For instance, most Classics/Universe Autobots have clips or pegs that allow their weapons to be stored on or even in their vehicle modes. Occasionally, this sacrifices some of the distinctive look of the accessories (a sacrifice that third party designers often use as an opportunity to cash-in).
But it looks like the days of having to keep track of extra Transformer body parts are being restricted solely to Kre-O collectors.