The most popular theme for thematic collectors is birds, and it's easy to see why! Many thousands of stamps featuring birds have been issued since 1845 (one of the famous Basel 'Doves' sold a few years ago for many thousands of pounds and people were shocked as it was for a thematic collection!)
The range of stamps featuring birds is so vast that many collectors narrow their collection to specialise in ducks, birds of prey, penguins etc etc.
The UK has just issued a superb set of 10 first class stamps featuring owls. Perhaps now is the time to start collecting these issues?