Some ideas for the assets.
In the Theme Park you can add modular attractions like a horror house whit some modular hallways etc… For the user can create the level like they want, some food stall, and decoration from a theme park like poster of shows or something like that. Bathrooms every theme park have their own, a big entrance (modular) to the park whit some big sign and (where you buy the tickets i dont know the word in english sorry), control room whit monitors, maybe and abandoned function site where they made espectacles. And i think some very very different decals would be awesome other assets only have 3 or 4 and its very repetitive.
For the playground you can add some dead trees, swings, toys of childrens, modular grass labyrinth would be awesome, rusty fences.
For the country side unlike of the city some old houses whit old stuff, old furnitures, ranch, farms…
Every asset in unity store have the same cars, maybe some differents cars would be better like bus, taxi, ambulance, normal cars but different models, sports cars, sedans etc… Garbage truck, trucks in general, and as the previous post says every car need to have different parts you can rotate move etc…
Roads of diferent types like cracked road, speedway, parking lots, sand road, advertisings billboards, bridges and tunnels etc… Whit Parkings lots i mean the entrance to the park can have a big parking whit few wrecked cars.
And the interiors, this is the best part for me every interior in every asset of Unity Store for me seems prety “dead” empty, need some props like food, tvs, many furnitures, store props like clothes, benchs, shelves, cash registers, maybe office stuff, tables (i dont know how to say te cubicles where the people work), pcs… Some bar props, bar counters, shopping mall, Parkings a building not only parking lots in the street like the park, museums (that can be easy made) i mean its easy a big modular building whit some painting where you can change the texture and some statues, cinemas, Schools, cranes… Variety not only the same building and houses.
And other different ideas, an airport whit old planes or a plane crash i mean its a post apocaliptic world you can see a plane crashed in the city or the mountains, skycrapers whit open interiors (modulars) then you can create the interior or old skycrapers just for decoration.
I hope this helps i try my best and sorry for my bad english