Many people see the companion as a love interest for the Doctor. I think the "love" aspect between Rose and the Doctor was a bad move on behalf of the producers. The role of the companion is a way for us, the audience, to journey with the doctor and experience his way of life. If we couldn't identify with the companion, we wouldn't watch the series. Many people could definitely see themselves as Rose and would say/do many similar things as she did. Martha is a different kind of companion - and one whom I identify with more than I did with Rose. She challenges the doctor and questions his lifestyle and moral choices. She has more of an understanding about how his mind works and appreciates her respsonsibility in certain situations ("Human Nature", "The Family of Blood", "Last of the Timelords"). I think - given one more series, Martha's character will be let loose a little and she would get to do even more. Catherine Tate worries me. Rose was simply along for the ride, Martha challenges the doctor and both benefit form this, Donna Noble is a major barrier for the Doctor. She argues with him and does not approve of his way of life. Why oh why is RTD making her a companion? I am sure that there are people out there who would treat the doctor in the same manner but Donna is a complete role reversal of Rose. At least Martha walks a fine line between the two. A future companion which would interest me greatly is Sally Sparrow. I truly hope she becomes a companion after Martha and Donna have left the scene. She is more like Rose than Martha but has a degree of intelligence and understanding about how time works as demonstrated in the fantastic episode "Blink". If you watch closely, there is a look in the Doctor's eye as he and Sally meet which suggests he might be thinking "She's pretty good at this wibbly wobbly timey wimey stuff - might make a good companion!"