We really don't need any one particular thing over another. But if you're looking to fill a gap, the Characters page will show you what's already out there, or you can use the census command in-game (help census).
We strongly recommend that your main character be either a marine or pilot. The main focus of the game is the war, and being in these departments will make it easier for you to get involved in plots and RP.
When you're ready to get started with your character, check out Character Creation.
See Timber Wolves for details about the ship and crew.
Note: To help spread around RP and not have one person doing All The Things, characters are only permitted one specialty. Viper pilots do not fly Raptors and vice-versa (except perhaps in the rare emergency situation).
All marines are Riflemen first and foremost, and that will be their primary role on most missions. But there are a variety of secondary specialties that can provide color to your RP and may prove useful on certain missions.
Note: To help spread around RP and not have one person doing All The Things, characters are only permitted one specialty. There are no tech/gunners or recon/medics for example.
Any marine may also have extra training (via background skills) in Vehicles, Communications, Languages, Parachuting, Diving, or a particular type of environmental warfare (Arctic, Jungle, Desert, Zero-G). These are by no means required, but can come in handy and explain why your character was selected for the Timber Wolves.
There are a handful of support personnel assigned to the Timber Wolves. These people mostly stay on board the ship, but may be sent along with the ground forces in special circumstances.
The game has been designed so that here are no "special" roles (e.g. department heads or special jobs); everyone is on equal footing. There are also no quotas limiting us to a certain number of Viper Pilots, or Captains, or combat engineers, or whatever. However, the following concepts are not allowed.