IDK, I tend to only use the free classes and I have a lot of fun with them. Most of them are highly underrated. Gatekeeper has a pretty great skill tree, so does Hunter (my favorite) and Escaper.
I think the best thing you personally can do class-wise is to switch it up. Try a new tactic.
As far as players go, Pat and I always feel like the people play if you play. In other words; If you want more people to be on, play more. Encourage others to play. Be nice to randoms. Teach the noobies.
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ZAM is a mod for a game that was released on November 9, 2010. That's nearly 7 years ago. The player base isn't what it used to be. That said, I still find myself playing with 6-10 other people a few nights a week. On weekends that number can go up to 18 and fill the server on occasion.
If you want to bring in more players, spread the word. Come up with fun activities to do while playing. Maybe setup a "Bragging Rights" weekly knifing tournament. There are always options.
~Kevin