Is there a reason you want to drop herbalism? Just asking, as at least on my server, I make around 35-45 gold per stack of 20 of the new high level Outlands herbs while stacks of fel iron ore and adamantite or go for about 13-20g depending on the day. I make most of my money from herbalism. You just need to skill up your herbalism a little more to be able to pick the higher azeroth herbs that sell for quite a bit still, such as dreamfoil and mountain silversage.
If you really do want to switch, what are you looking to do? Ie craft items for your own use, other people to use, or just keep doing a gathering prof?
Skinning is your only other option for gathering, and it is a great steady money maker. The only thing I find is that once I hit the lvl cap, ie 70, I no longer want to take the time to go kill beasts for long periods of time to skin them as I no longer get experience for doing it.
With mining, the complementary trade skills to it are engineering (not great unless you pvp a lot), blacksmithing (not so great for a hunter) and jewelcrafting (market still flooded with new people doing this since the expansion so profit margins are small unless you get a rare pattern). Yes, any of them are viable and can be fun, but if you're looking to just make money keep mining and herbalism and just level herbalism up to what it should be. It is a pain sometimes to change between tracking herbs and ore, but I think the money you make for makes up for the slight hassle.