If my memory is correct, in the TV-series Malone's fair maiden is waiting for him (or probably not) in New York City and his longing is directed a little bit more to the west, across the Atlantic. My personal opinion is, that his longing is directed much more to a fellow stranded character whose name starts with a "V". No matter where he will eventually turn up, I agree with you that so far he is the most expendable character. Rather pale (as a character :)) and with nothing much to do. Then again, he is a perfect reflection of most people. Just see him as an anchor of mediocrity in a world made of camp. :)
Maybe there is potential for him, though. Malone is probably the character that could grow most, because he starts at such a medium level. As I said before, I thought it was a mistake to change actors between the pilot and the regular series, but maybe the producers had no other choice.

As for Roxton, I like the character and wouldn't want to see him go. And I don't think that Challenger could be a real replacement. Getting rid of Malone would be possible, dropping Malone AND Roxton would disrupt the balance of the group.

All in all, maybe it's best to leave the group as it is, define the characters and their reltionship more closely and do something substantial with Malone. No, not killing him off. :) Yes, I can hear you thinking!