Make a bulletin board where players can give missions to other players. Make some templates like these:

Escort:
Give start/destination port. Now if the one offering the mission gets there, the one taking the mission gets his money. If not he's got to pay a penalty to the other one. He also has to stay near him.

Assasinationn:
easy, right? There need to be some information about where to find the victim

Espionage:
Find out where a certain player is. Best through station com.
Find out what is in a given system.

Trade:
Take this cargo there (within this time)

What I really want to see is group trade missions. Right now getting your trade level up is painfully slow at the current levels. Just my 2¢ worth :-)

Personally I don't like this experience stuff. I'd say regulate access to equipment through money and standing.