“Wannabees” is a term coined by the author of Making Sense of Creation & Evolution to identify fossils which are intermediate or transitional between major groups of biological organisms. For example, if reptiles evolved into birds, we should be able to see the fossil of a reptile that wants to be a bird but isn’t fully there yet—a wannabee.
This is a key scientific problem with macroevolution—there just aren’t any certain wannabees. In his “Origin of Species…” book, Darwin said there should be millions of transitional fossils (wannabees). Yes, there are a few candidates which are proposed from time to time, but they are usually based on some similarity of structure (called homologous structures). This is circumstantial evidence which can also be explained as the result of intelligent design.