I'd level the alchemy across the board.
Which then leaves one profession per toon, that can be something else.

It costs 2.5k to 4.5k to level an alchemist entirely through AH purchased herbs.
By buying when they're cheap, instead of what you need for a toon who needs whatever now, it is closer to the lower end of that scale.

Even with the price of gems having fallen, if you have a JC toon for epic cuts, you're looking at least 100g per transmute.
Even without a JC, you should make 75g a transmute, just selling the transmutes raw gem on the AH.
Prices will vary with server of course; some cuts would make me only 50g (I don't cut these gems) but yesterday Runed Cardinal Rubies were selling for over 200g (which is 175g profit per gem).

Even assuming only 50g per gem, and 4k to level an alchemist... they pay for themselves in under 3 months, after which you're making a profit. At my server prices, its three weeks or so before they start to show a profit. With dailies, you can make 1,000+ gold in an hour per team at 80th, so put out 2 alchemists a week.

With a new arena season, or a new raid zone, gem prices skyrocket to at least 150-200g profit per gem. I'm sure Cataclysm will have cycles too; you could look at tradeskills purely as getting ready for the next expansion.

It might be an idea to have one of each production skill as well. But that's a fair bit of work. Jewelcrafting, Blacksmithing, Leatherworking, Tailoring, each of these will likely have combines (or cuts) which can make a fair bit of profit.