here's my thought, starting from broken's suggestion.
Instead of "upgrade", how about further specialization or creative alternate abilities. That way the original classes still get some use.
For example, the Guerilla - An ambush specialist. Your first attack on any enemy does bonus damage. Barrels do more damage, except to you. Trait list would probably be based on the Marine, but at least one swap - MFA instead of MAD.
Edit: Although we have gotten a bit off-track from the Masterless suggestion....
In other words, he's the doomguy we had before all these Technicians, Scouts, and Marines started clogging things up with their Gun Kata and their Ammochains and their Malicious Blades
Cateye was in before the master trait system though. >_>
Ultimately, though, I think this ties in to the criticism of the "upgraded" classes - very few people would bother with it if it didn't have some sort of bonus or advantage.