I myself would have chosen Kate Hudson for the part of Goldberry, though I cannot for the life of me think of who I would cast as Bombadil. I shall just stick with my opinion that Peter Jackson was right in not including old Tom, jolly as he is. Robin Williams would have been funny, to be sure, but he certainly would not have meshed well with the atmosphere of the first film. I like picturing Bombadil in my own way, and I don't think any actor could ever capture him for me.
I'm not sure why, but I have been picturing Paddy Considine as Eomer recently... perhaps he would have made a good Erkenbrand, assuming he could pull off the proper accent.
Well, there's my slim contribution to the topic, take it or leave it.
(honestly, must the pipe-smoking smiley face really come with sunglasses? It seems rather silly.)