Swashbuckler~ Fire (they both can cause powerful damage over time and have lower health) Musketeer~ Balance or Storm Witchdoctor~ Death Buccaneer~ Ice Privateer~ Life (they both can heal)
I was unable to give details on why I think Musketeers, Witchdoctors, and Buccaneer have the classes that they have since I've just started on a Musketeer and Witchdoctor and haven't even played as a Buccaneer yet. But that's just my 2 cents