I think the best time investment an Alchemist can make for Cataclysm is to stockpile flasks. We heard this week that heirloom items will be allowed for Cataclysm leveling, which means most players will zoom right through much of the Vanilla content. But the leveling curve gets worse at 60, so I think folks will be open to paying more for a flask. They'll want to get through Outland/Northrend quicker, and if they're prone to dying the persistent nature of flasks will be appealing. So my primary recommendation is to sell flasks. Here are my top choices for this category:
Level 50-65: Flask of Supreme Power
As you can see, the level 50 flasks do not include a melee option. Unfortunately there are no appropriate flasks for melee dps from classic WoW, so those folks will have to go with elixirs for their chemical enhancement. At level 60 there are two choices for caster dps because spellpower hadn't been combined into one stat yet. You can just pick one flavor and run with it or try selling both. I think the biggest obstacle to supplying all of these items will be availability of recipes. Depending on how long you've been an Alchemist, you may be missing some of these recipes. In that case just make the ones you have from those I've listed.
Grads: Elixir of Brute Force | Elixir of the Mongoose | Greater Arcane Elixir | Purification Potion*
I would categorize the Elixir list as medium priority - dabble here if you want but don't feel obligated. You might just pick a few items off this list that use up old herbs you have lying around and leave it at that. The only strong candidates I see are those melee buffs that weren't covered in our flask list. This would be the "Grad" elixirs such as Brute Force or Mongoose. Also, I've included a couple of cleansing elixirs in here, marked with an asterisk. It's been so long for most of us, but back in the day poisons and diseases often lasted 30 minutes. If this doesn't change in Cataclysm, players may be willing to stock cleansing potions to get rid of these nasty debuffs.
The bottom of my list is the health/mana potions. I'm not going to list each variety, I'm sure you can figure that out. I don't really think this will be a strong selling item because these pots also drop as loot. There may be some sporadic spikes if loot supplies dry up, but I really don't think this will be a reliable sales category. If you want to give it a go, feel free. Me, I'm just saving up the ones I'm looting in old world dungeon runs. No sense actually farming the little boogers, hehe.
I would like to make an honorable mention before wrapping this up: Mercurial Stone. This is a trinket that can be sold on the Auction House. Turns out it's a pretty decent trinket for those who don't have crafted options of their own (Engineers & Jewelcrafters). I think the Hit rating will be very appealing to leveling players, a stat that tends to be hard to get on leveling gear. I plan to make a few and see how they sell.
That's all I've got for Alchemists, if you have other suggestions you'd like to share please leave a Comment!