For more realistic and challenging, Ferram Aerospace and RemoteTech Relay. One makes the aerodynamics more realistic in flight. The other makes satellites have a use for communication. Anything unmanned has to have either a powerful enough satellite and line of sight to KSC, or line of sight to another satellite. As far as must have, at least until the next update, Kethane was doing the resource thing when it was still just a long term goal of the devs.