Actually, there are more elements with no friendly elements - next to Life all of the unifying elements. Furthermore, I have to admit, that I don't like it, that several instances forbid the knowledge of some spell lists - one of the very things I like about the specialisation feats is, that you can become a specialist of everything (except Create
) without sacrifacing knowledge. The other thing, I like, is, that you don't start as a specialist, but grow into one. The above mentioned restriction validates this wonderful idea.
As a houserule, I'll won't require the ignorance of the opposed elements, next to the addition, that elements without friendly elements need then the knowledge of the opposing one. This allows to become Guardians of Life and Death simultaneously, but you have still to use at least one feat for each element, so it isn't really a concern. Hmm, I see, that no unifying element has an opposing element. In that case, every unifying element has to be known.
Another discovery, I just made: Earth, Crystal and Metal don't have any opposing elements? Because they don't deal energy damage?