Allow households and lots to be transferred between saves

You can already do this to some extent. I did this to bring households together in one save for the same reason you mentioned. 

You can save a household to your local library. You can upload them to the gallery but your library should be enough if you want them on the same pc. To save households you need to go to the world screen, household manager in the top right corner. Find your household, select edit (pencil icon). This takes you to CAS where you can save them.  then place them from your library in another save. You can do the same with lots. Lots can be saved from build mode. 

Note:

• Inventory (sim inventory, household inventory and storage boxes, bookcases, fridges, toy boxes,... ) does not transfer. My workaround is to place all the objects I want them to bring along on the floor (for example in a basement) before I save the lot to my library. Then put them back into the inventory in the new save
•  Relationships between households need to be redone 
• Traits, aspirations and skills will transfer for households. Not sure about money. Cheats can help to set them back if you want to.