I think you’re on the right track with increasing the credit earnings. People only have a limited amount of time, and would feel rather gypped if it took them too long to work towards a character.
I also like the idea of adding in more missions; one fellow suggested 9, but I think 6 would do. Increase them to 700 each, leaving you with 4200 assuming you complete all of them. This change would give you a bit of extra credits to spend on trading up; as it stands right now, if you want to see your credit count rise, you have to fore go trading up. Well, at least I do.
Try to structure these 6 missions so that it encourages people to focus on the objective, or explore new classes. You could divide up the existing Support EXP mission until several missions for specific support classes, so people try new things. You could do the same with combat XP. Though the game is primarily focused on objectives, you could have weapon missions, such as “Get X amount of kills/score using Y weapon,” which isn’t necessarily class specific, though I’d limit this mission to weapons used by offensive classes so that support classes don’t neglect their duties. Another possibility is “Kill X amount of Y class.” For example, Kill 30 Aura’s, encouraging people to focus on destroying healers. “Destroy 30 enemy deployables,” so mines, turrets, healing stations, etc, keeping people focused on productive activity.
I tried to come up with missions that would encourage team play, rather than lone wolfing or neglecting duty. Also, there’s always the possibility of somebody slipping off to some quiet server with their buddy and farming this stuff. That’s probably something to consider. However, I’m sure measures to prevent those sort of shenanigans will be put in place. As with all things, it would take time to develop and implement (or even just implement an existing system), but I believe it’s well worth it.