Engineering is overpowered
Engineering is by a very significant margin the strongest profession in WoW classic. This means a lot of players will have engineering. This might make you go “But Lazy, that means they can craft all the items they want!”. That’s true, but players are lazy, they don’t want to put in the effort. I sold an absolute ton of engineering items in WoW classic. Anything from ammo, sapper charges, grenades. They all sold really well, and they are just as good in hardcore.
Consumables is where it’s at
Generally speaking you want to focus on the consumables. In Hardcore the grenade type throwables that CC mobs are particularly useful, as they can save you in a pinch. I do expect sapper charges to be less popular, as the damage downside is riskier in hardcore, but overall I expect similar results to classic.
Good recipes at any level
You will find good recipes at any level. even at just skill 105 there are plenty of profitable recipes, indlucing the option of crafting rough dynamite and just vendoring it for a profit. That being said the end-game or close to end-game recipes are where the true profit is probably at.
My top recipes from the original game include Thorium Shells, Thorium Grenade, Goblin Sapper Charge and Unstable Triggers All of these are either very useful for max level characters or used in recipes that are useful for max level characters.