Since there are only 7 towns to pick from with store worlds excluded, I've been looking into community lots in each town. If you want to preserve the integrity of each town, then the most conservative option is:
You may go into Edit Town only to add either the Book Corral or the Penningway Bookshelf to the libraries in Twinbrook and Bridgeport.
If you want to take Sunset Valley, Appaloosa Plains, Moonlight Falls, and Starlight Shores as the norm, then in addition to the above, players could add:
To Bridgeport -- an elixir shop and an equestrian center.
To Twinbrook -- an elixir shop.
To Isla Paradiso -- an elixir shop, an equestrian center, and a performance park.
(To save space, I would allow players to add an elixir cash register to the consignment shops, but that's just a suggestion.)
If you want to allow players maximum flexibility with regard to skills and careers, then state something like:
Players may alter their towns however they wish in order to include bars, the film studio, the equestrian center, the showtime venues, the elixir shop, the gypsy wagon, resorts, the bot emporium, the bot arena, and the observatory.
Edited to add: The reason I refer to AP, SV, MF, and SS as "the norm" is that they already contain a salon, consignment shop, fire station, elixir shop, equestrian center, and performance park.