It's can be very straightforward, really. We can start by identifying 7 good types of crisis configs, and their strongest gameplay traits, purpose in game, play mechanics, etc. Then we can flesh characters out of them by distilling and magnifying the key traits.
Eg:
Fireknife BG --> med-long range, general purpose, high strength, decent volume of fire --> poor BG, but good supporting squad leader + leadership abilities.
Helios BG --> short-range, heavy firepower, esp. vs heavy infantry --> cc "specialist" with a "counts-as" powerfist (FB), good against MEqs, TEqs, MCs
Sunforge BG --> Short range, fire-based weapons --> Demolition specialist, FB+Flamer, Bonuses against Hordes and Armor
Deathrain BG --> longest range --> Sniper
Firestorm BG --> med-range, high RoF --> generalist, but good against infantry
^By no means final or absolute. Just throwing out ideas to get the ball rolling.