Whenever player encounters any difficulty. It is the time to sell your product and consume their resources.
1． Reduce procedural control cost
In Millionaire city, when you build your house in the lot that you haven’t bought, the interface will remind you how to buy the lot to build your house. It is good that tell player the procedure to complete that action. But I think it will be better that the interface reminds you if you want to buy the lot immediately that the house needed to build on. So that the player will not need to convert to buying lot interface, which will add the control cost.
2. Need more efficiency
In Millionaire city, you rent your house to customer, but you have to sign your contract with the renter. You can sign one by one, but you can also employ a contractor to help you to sign contracts in a time in a range which you can buy. Even when you offline, the contractor will automatically act. The rent contract sign interface will be better to include contractor recommendation icon. Because Player may choose a contractor when they are offline.
Similarly, the rents collection also can be Automation by choosing a rents collection contractor.
3. Need power
In the Plants V.S. Zombies, the icon of gem bomb will automatically remind you to use it through flame animation, when you are in a fierce battle. That is good that to recommend the bomb when player may needs it.
I think we can learn from that method and modify it to recommend new plants. Like the level unequal plants which may be very helpful to that level. So player may buy those plants in this level otherwise they will not buy them in other levels.
There will be more helps that you can provide to your players which will not confined to my mentioned above. When player have difficulties or want more conveniences, or want more efficiency that are the chance to monetize them.