my suggestion this time, is to our dwarves start big entrerprises, like have more than one shop, and buy more shops in different towns, and create new brands, like "Dwarv'os, your Dwarven morning cereal!", and so, they can own the workshop where they produce his products(the player choose what workshop will be buyable), and them, some of our dwarves can turn into tycoons, with really big quantities of money in account!
- Have more than one shop: Expanding beyond a certain limit would necessitate the original dwarf to delegate the responsibility to another dwarf, who would be perfectly capable of starting his own shop, since the tools of the trade are relatively cheap.
- Buy more shops in different towns: In medieval times, different towns had different coinage, different laws, different customs, different language, different families, different nobles, different measures, etc. Lack of a nationalized trade structure would severely cripple this.
- create new brands: They did care about their brand, but brands would be tied to a region (eg. Parmesan cheese) or a family/particular shop (eg. gobelins became the standard name for wall tapestry). Branding didn't happen because of the lack of mass media, the slower pace of the economy, and a social structure centred around family ties and regional affiliations.
-they can own the workshop where they produce his products: That could happen, though to be able to sell them as his products, he would still need to be involved in the production process, even if only in the design or the finishing touches. Even so, actual quality should go further, for dwarves at least.
- tycoons, with really big quantities of money in account!: Banking was rather undeveloped in 1400 AD, and I presume Toady prefers to stick with the period where they carry bags of gold around, guarded by men-at-arms, for dramatic reasons.
So in general, I think accumulating big fortunes should happen by having the title and the rights of territory (nobility), trading (merchants) or actually producing stuff (master artisans). Actually owning shops they don't use themselves should be avoided to avoid making it into a positive feedback game of monopoly, with one dwarf owning the world. This will be balanced in-game through guild actions, communication difficulties (the guy running your shop in another town will effectively run it for himself, and that's not profitable for you) and loyalties to the local groups and families rather than contracts and property rights.