In general, I tried to only mark maneuvers that people would find useful. One of my big goals was to make sure people didn't get completely overwhelmed with choices.I'd be glad to see them!
Once you've played the base game, however, I'm fine with adding more complexity in an expansion. Orbit to fly-by maneuvers are something I keep thinking of adding.
As I already said I started looking for them to quickly, easily, and repeatably test aerobraking with the same component which might not be good to allow, but they'd also be useful to move from Venus/Mars to the outer planets without returning to the IPT.
If the extra cards can be squeezed into an expansion I'd be happy.