I'd like to see something that allows people some time to trade and enjoy their items before it's time to move on to the next set. That "other" restaurant game was fun when we had time to barter for the items we wanted and then had time to enjoy our new layout - at the very least two weeks before a new theme was introduced; monthly being preferable.
If there are going to be items to buy with cash, they should not be expensive. Someone shouldn't have to spend more than $20/month to have anything they could possibly want in their restaurant. (It should at least cover the amount of purchasing a complete set of new stoves and whatever appliances need to go with the set.) The more someone can buy with $10, the more they are apt to be willing to spend that type of money on a consistent basis.
The higher you charge for virtual items, the more people start balancing the actual cost of "real life" things against game items and figure out they could spend the money on something for themselves in the real world. If it's a smaller amount of money, they won't think as long and hard about spending it.
This also keeps in mind the number of countries that won't be able to purchase cash credits who will only be able to trade with others in the game to get what they want.
There should be a trading system that allows someone to sell their items by entering how much they are selling the item for and the person they are selling it to so that amount can be deducted by the game from the other trader's account. There should also be a way to keep records of what was purchased and sold that way.
There should be a conversion of cash items back into cash credits so if someone CAN'T get access to cash items, they can barter until they have enough to sell to get cash credits. It doesn't matter if the conversation rate is low, as long as people who can't get access to purchasable items have some recourse to work for what they want.
As far as recipes go, they shouldn't be hokey. It should be planned out so what is being served and the ingredients needed to make the dish make sense. Again, people should have time to collect their ingredients as not everyone gets a chance to log in every day. I'm a big believer in gifts to help people top off their dishes.
You should have the ability to own more than one restaurant and build each one up level by level, again with the ability to move inventory between shops as any chain restaurant owner would.
There should be a garden and the ability to have a gardener character to tend to it after you've fulfilled some level of gardening. Several spa facilities have their own fresh gardens to grow their own food. It should give them some type of badge that allows them to call themselves an organic restaurant.
One of the MOST important things is there should be some type of "chalkboard" feature that allows you to put a long message where people who are going into your restaurant can see it - possibly an item that you can have more than one of if you want more than one board. They would be killer for quests because you could put them in the restaurants with further instructions.
Those are some ideas off the top of my head...