Hi Lynn…
I appreciate and encourage your genuiine interest. You ask a great question. I have to confess that, to an extent, I really don’t know how to offer you a precise definition of what a novel body plan is. I will say that the more distinclly a body plan can be identified as a novel body plan (in other words, the higher up in the taxonomic order we find a particlular body plan), the more confident we can be in saying that it’s so. Encyclopedia.com offers the following definition, which is very much in line with others I have seen:
A body plan is a group of structural and developmental characteristics that can be used to identify a group of animals, such as a phylum.
Thus, the example of wings becomes a great example. Creatures that have fully formed, functional wings are said to have evolved ultimately from creatures that had no wings according to the grand narrative of evolution. That’s fine, but according to evolutionary theory, every step along the way from the wingless ancestor to the extant winged creature must confer a functional advantage ( I realize that those who invoke drift might object here, but drift is a different subject ). Therefore, if we want to say that it is absolutely true that extant creature x has descended from ancestoral creature y through an evolutionary process, we should be able to explain a detailed evolutionary genotypic as well as phenotypic step by step functional adaptive continuum for wings (or any other novel body plan). If and when such an account is ever forthcoming, then we can analyze that functional adaptive continuum to see if we can gain any insight as to whether that process was the product of purely natural processes or intelligent agency.
Keep in mind that there is no such detailed adaptive continuum currently avaiable. Despite this, there are many folks in these forums - as wonderful and well meaning as they may be - who would forclose on the possibility of intelligent agency a-priori.