Retyping is kind of a choose your battles sort of thing. For example, while you could replace Roselia's roses with boxing gloves to make it Fighting, trying to remove Ninjask's bug features would be impossible without almost completely redrawing it. This is true for the majority of Pokemon based on birds, bugs, plants, and sea life, though there are some exceptions; you could fairly easily make Cacturne look like a Normal-type scarecrow. The key to retyping is replacing features rather than removing them, which I think is the problem you're running into. Normal would probably be the most difficult to retype to though, since they don't really have an equivalent to leaves or rocks... They're just normal. If you think about the general shape of the part that you want to replace, however, you should be able to some up with something (a ribbon, a balloon, a tuft of hair) that will work.
As for the WPC, yes, I believe that the goal is to come up with an entirely new type and to retype a Pokemon to that.