A) It would be nice if users could add user defined properties.
The number of properties is too low and this results in overloaded meaning of properties and any action could come up anywhere (this is way too strong, Bliss makes an excellent job considering the constraints). Players have limited control during the game by building properties. In some sense this is good, because it adds random spice to the choice. But it gets problematic if less mainstream actions get loaded. Also it denies the players the choice to direct the game.
Adding the capability to have more properties and keeping the basic pack to more generic properties would be very good. Pack developers could map a few generic actions to the generic properties and have specialized properties for specialized topics. Assuming e.g. I develop and Ice Cream Parlour action pack I would map a few actions to the Sweet Shop and most others to my Parlour.
Possibly it would make sense to allow the selection of properties at the beginning of the game (similar to toys). This would also reduce the need to switch on/off action packs.
I understand (from one of the other posts) that you use some reasoning to figure out which property is available for buying. This should be tied to a threshold number of actions being available to execute - and not as it seemed from your post to the availability of certain toys (probably requiring some hard-coded logic).
B) It would be nice if properties could be sold
To get back part of the money or make space for a different property.
C) It would be nice if properties could be further developed
Not sure whether it is wise to add this dimension, but let me explain the idea:
Variant 1: A property would - by default - have a level e.g. 5. To get to the actions at the higher levels 6-9 further investments would be required.
Variant 2: Investments in a property beyond buying e.g. a StripClub would result in a local boost of the game level (but never exceed the limits of the players). When subject enters the field and the subject is at level 5 then the subject would be treated as if at e.g. level 6.
Last but not least: Excellent game, well balanced design choices.