Alchemy was my most consistent goldmaker in all of classic, and with BC still running the old AH overcutting will likely remain incredibly strong.
Consumables always sell
Alchemy consumables have always been a staple of goldmaking. They give you significant power increases, and they are consumed on use, so you need to buy new ones for every new raid. Alchemy always give access to the main consumables and this is also the case in the Burnind Crusade.
Classic had an absolutely crazy amount of different consumables that could be stacked. in TBC the consumable system is simplified with elixirs beign lumped into two categories: battle and Guardian Elixirs. You can only have one elixir from each category. In addition flasks now count as both categories at the same time. Since you retain the flask buff through deaths they will likely become the meta pick for everyone, unless they turn out extremely expensive.
Reputation and discoveries
Most of the recipes for Alchemy come from other sources than your profession trainers. New in TBC is the discovery mechanic where certain recipes can only be learned as a discovery from crafting other alchemy recipes. All of the recipes from discoveries can be learned by crafting any TBC alchemy recipe except the cauldrons that you can only learn from crafting the specific major protection potions. The most important discovery recipes are of course all the flasks, which will likely be the meta consumables, unless fel lotus is just too expensive.
There’s also a good chunk of recipes locked behind reputations. The most relevant reputation recipes will likely be the meta gem transmutes from Cenarion Expedition and Honor Hold/Thrallmar, as well as the major Nature Protection potion from Cenarion Expedition.
Flasks or elixirs?
Battle elixirs are the elixirs that give offensive stats, whereas guardian elixirs typically gives defensive stats. Flasks count as both and generally give much stronger bonuses than battle elixirs. On top of that the effect is not lost on death. I expect flasks to be the go to, unless they are prohibitively expensive. The harder raids turn out to be the better the permanence of the buff gets, but even with few expected deaths the buff is so much stronger that most players will want to use them.
New in TBC is the addition of various DPS increasing potions. This has been a staple in the game since then. I’ve not done enough research on boss encounters to know if there will be bosses that require protection potions, but for any boss that does not these 2 minute cooldown DPS increasing potions will be incredibly good. I expect these potions to sell quite well. Most of the recipes are world drops that can be traded freely so be prepared to search the AH diligently to snag them up cheaply.
Stack sizing and overcuts
Utilizing a wide variety of stack sizes is key to making gold with alchemy. All throughout Classic I made 10-30% more than the current cheapest price by posting my potions and elixirs in stack sizes from 2 to 5. By always having the right quantity someone might want you can very often get sales from lazy players who do not want to click, or from players using the default UI for the auction house, which is frankly terrible.
I’ve made an initial setup for TBC alchemy based on my stack sizing from classic. It does not include crafting operations, as I do not know how sale rates will look yet. Keep in mind that market value and crafting cost is usually a bit out of whack early on in an expansion, so at the absolute beginning I would strongly suggest sticking to hardcoded gold values or posting directly with auctionator or a similar addon.