Well, if you plan on doing a lot of gathering right after the xpac release, you might be able to make some decent cash farming mats for the new tradeskill stuff. But with the sheer amount of players at cap with gatherers, it might be a pretty bloodthirsty and quickly diving market. I imagine on Gilg that the market for non-timed/rare spawn mats is going to start alright and then bottom out with a quickness, with the other gathered mats falling quickly down there as well. Largely it is going to depend on how the crafting stuff is gated and if you need obscene amounts of gathering/perception to even swing at nodes, which would make the first people to them the ones that set the prices. The upside is that crafters in general are pretty rich, so they will probably drop quite a bit of gil on easily farmed crap just so they don't have to run out into the field.
Since both the actual gathering classes are spread among the various crafting disciplines, you'll get a lot of mileage out of either one if that is your intent. But man, you have a lot of very mindless grinding ahead of you to get them to the point where they will be poised to make money with the expac. Even with max rest xp and leves and using the scrolls from the grand company, that shit still takes forever and is incredibly boring. I'm finally working on finishing off my botanist before the xpac, and the repressed memories of when I did Miner have definitely come flooding back.