I herb and mine mostly. Transmog would be great if you can make profiles to run dungeons. My favorite expansion to mine and herb is BC. With some of my mats I make Turbo-Charged Flying Machine. If you dont want to make something with what you farm you can just plop whatever you farm on the AH. Id recommend posting the items on a separate account, and at the least a separate character than the one doing the farming. Once in a while I farm WoTLK mats to make Mechano-Hog. I dont like making Mechano-Hogs just because i have to spend my own gold on the vendor items.
With my time botting I have found that I can make just as much gold in the earlier expansions. I can generally farm more items in a shorter amount of time, and the items usually sell for around the same as the current expansion items.
I have also heard great things about the auction bot. If you can set it up correctly you can have it snipe items and list for a profit. I feel like it is probably the safest way to make gold without getting caught also.