Well, considering how well balanced Valve made the current incarnation of TF2, I'm going to give them the benefit of the doubt that they can add these new kits and whatnot without fucking things up. Personally, I would like to see more variety in the game and more incentives to try out the different classes. Most of the time, the classes seem to pigeonhole themselves in certain types of roles. While this helps to keep the game balanced and emphasizes the team effort aspect, it does make some of the classes uninteresting for people. I'm used to playing aggressive and offensive games like UT or COD, so I heavily favor the soldier, pyro, and heavy. All the other classes really do not interest me because the way to play them effectively isn't what I like. So I would welcome more customization and achievements in other classes to help draw players into choosing a wider variety of roles.
Medic's my second most played class. Every team I've been in with at least a few medics. The problem at the moment is that, while they are strong, they're a little too boring, which is why most people don't play medics. Giving them more options and flexibility will definately halp the class out.