Lots of decisions to make - Mages are a must for multiple groups portage, save some time invite most of them to raid and just cast one port. Paladins are usually good for any tanking situation so you do not "need" to have another tank. Unless of course you want to try a DK out when you hit 55 .

gathering professions can be thought of as a way to feed you money, or a way to feed other classes mats to craft. Skinning is an easy one to lvl, Mining like you said is nice to have since it feeds 3 different professions. Herbalism sells very well right now too.

Just think about what you need to compliment what you already have. I'm sure anything you pick can help out.