I agree with your thinking here. I will add that if Rosa is selling potions, others must be also. Competition will limit prices that customers are willing to pay. Compete on price, and quality, and efficiency in completing orders.
In fact, I thought about the law of value in the beginning. But if we aren't competing with other merchants in more ways, neither have some contact with them, then the competition can be simplified to a bonus/penalty factor on the potion selling system.
On the other side, brewing potions needs her alchemy ability, so unlike the arena fights, she starts with a high alchemy level, so there may be no competition concept in efficiency and quality. Your first meeting with Rosa needs your reputations too.
Consider involving the player more actively in the process -- an order is accepted, with a deadline. To meet it, the player must help. Succeed and grow business reputation, more customer interest. Fail and get blacklisted, lose progress, maybe temporary lockout from a district. Business reputation being a new concept here - you need faction fame (existing) to gain access to buyers, but to impress them and make progress requires business fame. Maybe?
The order is a good idea, I can convert all the customers into different orders. Yes I'm doing subtraction.
Your fame influences the probability (or weight) of different tiers of orders. "Hints -- The more famous you are, the more likely you can establish business contacts with richmen and patricians."
Blacklist may not be a good idea, you can use this way to purify your orders into expensive ones at no cost.
Business fame, we use it to increase the max number of orders in a decade.
At first you have no choice, because order numbers are few, as the increasing of orders, clearly you can earn more money, but when the total amount of potions requested by the orders is beyond Rosa manor's brewing ability, you have to make decisions. Orders ignored by you slightly decrease your business fame, orders failed by you greatly decrease it, scaled by order tiers. Finish orders will increase your business fame.
I agree with you that we should involve players in the process, but I'm not sure if it's proper to give players the chance to provide Rosa with your potions.
If we do, the potions you provided won't make extra profits, so I will increase the amount of your gaining of Rosa's attitude in comparison, but it may as well motivate players to become a merciless potion brewer.
Luckily, in order to make Rosa more irreplaceable, I didn't give players the access to brewing new potions. Rosa has a monopoly on it, so even if players do so during the researching of new potions, the cost is players aren't making any extra profits, and it won't last forever, players will know how to choose when facing more profitable new potion orders.
Work backwards from the total amount of income you want the system to provide. I suggest aiming for it to approximately covers its expenses with perhaps 100 - 200 sparks profit. That's in line with other income sources at the moment.
Rosa manor rent is scaled by difficulty, in normal difficulty, the Rosa manor breaks even at the beginning. It's gaining less during investment, but earning more later on, I don't mind that player gain much money (at least enough to cover your rent, 400~500) in a later time, it's players' efforts, not completely automatically.
And in the hard difficulty, it's an extra bill at the beginning, in the end it can only cover rent in the Bull.
But, in my intention, if you didn't train Rosa, she provides you at most 50% of her income. But if you train her, at most 100% (50%+ Rosa['moral']) when she is completely devoted and she is sent back to manor.
By then, money should not be a problem if you send her back, of course, you are responsible for all Rosa manor expenses. If you leave her as your assistant, the tribute percent is still 50% (the rest 50% is provided by Rosa's devotion to you).
High-level thought is that this entire system is ultimately just a fancy new pony race or arena battle. It's a recurring income source.
As I said above, you don't begins with a low level of alchemy and enchanter, then learning through the competition. So I don't think so.
I think we need to explore the concept of rivals and antagonists. At the moment not much happens unless the player initiates it. We want the game to do things spontaneously, so the player has to react and adapt.
It's a huge project to design a whole AI system of other slavers, but it make sense.
Reached B+ guild rep? A rival appears, challenging you to trade slaves with him every decade and see who can make their slave reach B+ first. Or something like that... Maybe a rival sees you with Rosa and wants to take her for a spin. Refuse and something might happen... etc.
I raised similar ideas before, the guild holds slave-training competition (to check their members' ability), provide you a short time, see the final result.
The level of competition you can participate in is limited by your guild reputation, and of course, any leaderboard should serve for better or unique rewards.
If you have any plans on S++ or SS+ skills, they shouldn't be achieved simply through training, but through competitions, for example, the arena grand champion guarantees a S++ gladiatrix skill, which is a symbol of masterpiece, SS+ rank demands this. Emmmm, just an idea.