My ideal The Sims game would definitely involve a heavier workplace emphasis. It would also be online-only (because that could work MUCH better for The Sims than for SimCity). It probably wouldn't get a number, but rather be explicitly marketed as a side-game (kinda like The Sims Online, but not shit).
First off, there would be a brand new item builder. It'd probably have to be a separate app, but something specifically designed to work with the core game. You start with base templates for different types of items (chairs, tables, mirrors), and a fancy toolset for modifying or tweaking 'em. Maybe even add a "from scratch" mode that gives users parts and lets 'em put those parts together however they want. Want a 3-wide couch with two seat cushions and one connected back? Cool, you can do that. Want a fancy huge table with 8 legs? Cool, you can do that. EA can monetize this by giving a core set of parts to work with, and charging for additional textures and pieces. For example, maybe there's some floral pattern and rounded table leg you want, they're 50 cents each, so you can design your stupid ugly table for a dollar.
Next, I would totally rework buy mode. No longer is it something you can pause your game and do on the fly whenever you want. Instead, your sim has to actually go into town and go to a shop. For simplicity's sake, you can still see your home and drop stuff in while your sim is shopping (nobody wants to buy a bunch of stuff and THEN position it), but at least one sim has to actually GO shopping for furniture and the like. Why complicate it like this? Because it leads into my next idea, which is where things get interesting.
Users can run their own shops. In those shops, they can sell custom item designs. It's basically a built-in modding community right there; Joe 3DGuy can design his own fancy furniture and import it into the game. They can sell items for either simoleans or real money, their choice. If they use simoleans, they set the price, and their sim's daily takehome is based on what they sell rather than an arbitrary daily income. If they use real money, the player sees a chunk of the profits, and EA takes a chunk. This incentivizes content creation, and even incentivizes paying for parts in the item builder. EA is happy, player is happy. To keep the game from being completely ruined, maybe NPC sims will also buy some of their simolean-priced stuff.
Here's the catch: Since the game is online, all players in a region share the same clock, which advances whether or not they're playing. A sim would need to be at work in a shop in order for the player that owns the shop to sell things, but a sim can't work more than eight hours in a day (SimWorld has really strict overtime laws, and sims need sleep). If a sim is making enough simoleans, they can hire another sim to work different hours and pay a standard daily wage, but it's less takehome for the shopowner. They can also hire other family members in the same household to work for free, but this strains their relationship since they won't see each other as much, and means that person isn't working another job for more money or keeping the house clean. Teen household members can work also, but only for a maximum four hours daily, and not during school hours. Really, I see managing shop hours as a large part of this game.
Due to the nature of what I've described, a few things would have to change from previous entries. Aging would need completely reworked or even eliminated, but unlike with TS4 removing toddlers, there's actually a reason this time beyond "we didn't have time to implement it". An MMO-style time system would mean pause and fast-forward are completely removed, because all players need to have synchronized time (to make sales possible when a player is offline). The more MMO-like nature of the game would mean serious changes for households and individual sims as accounts.
What I'm imagining to solve the household problem is each player account starts with one household of up to, let's say three adult sims and three kids. Since sims act as workers for shops (or other jobs; we're not eliminating those), the sims themselves become a commodity. You can move your sims out to start their own households, but there's a financial cost to the household as they will take some of the money with them. This also carries risk as the more homes you own in a region, the less room there is for other players who might want to buy your stuff (a non-issue for people who are just playing to play). Single sims can marry NPCs who they meet in public places (think downtown in Hot Date), or sims owned by other players (offer/counter-offer system involving trading sims for simoleans, so the player who loses his sim isn't screwed).
The best part is, by relying on user-generated content, I think this game could be free-to-play without having the usual free-to-play pitfalls. Think something similar to Team Fortress 2's monetization. I could definitely see a game like this being fun for players, profitable for enthusiasts and "professional" players, and downright lucrative for EA.