Great idea, this NPC would be very nice to have with the Dryad around. Could it be possible for him to give wrath potions to players in a world with crimson, and rage potions to players with a world tainted by corruption? That would be a pretty neat way of getting some valuable potions that are unobtainable otherwise. It might be nice if she/he could also sell the seeds required for each alchemy plants similar to the Dryad's planter boxes?
I don't really think that's a good idea -- I don't want to remove the need for content unless it is so basic like sand (which is why I put the bottled water in his inventory). I don't want to completely remove the need for gathering seeds or the Staff of Regrowth and its perks.
Please try to keep in mind: AFAIK,
there is no way to limit a quantity that a shopkeeper sells which means if you put something in his inventory to sell, he will sell an
unlimited number of that item.
If there were a way to only allow 1-5 potions to be sold at any time, then I would have made a suggestion that he could sell a few basic buff potions, but the fact that anybody with some plat could buy chest-fulls of these things whenever he had them up for sale makes it a deal-breaker.
So no, I wouldn't change my suggestion to have him ever having buff potions in his inventory for that reason.
"One Tier Behind" Healing/Mana potions are OK -- Healing Potions are limited by a potion cooldown and it is rare that you don't have 30 on you at any given time, and Mana potions come with that whole magic sickness thing and people who use them commonly do a ton of them anyways.
As for the opposite biome potions, those would be included in his random quest reward, in fact if you wanted to, if possible, you could give it a higher priority, perhaps make them a
separate class of potions, something like this:
...
Drop #12: Spelunker Potion
Drop #13: Gills Potion
Drop #14: Rage -or- Wrath, 30%/70% split, the 70% is the one that DOESN'T appear in your world.
...
etc.
That way, players on a certain world have a chance of getting the potion that can't be made on their current world. Of course, it would still compete with all of the other potions. You could probably group up other potions too, such as grouping miner and builder potions into a "non-combat" category to clean the RNG up, but that's more a programmer's thing; I don't have the source code and I don't know how it works, so someone who does know would know better lol.
I'm only coming up with a concept here.