Hopefully it's a long way off so that it will be enough of an upgrade because let's face it, it's always an initial downgrade due to all expansion material you'll have to miss. However, I feel like there is real art in deciding what to launch the game with when knowing you have to leave plenty out due to time constrains and budget.
So here's what I'd suggest for the sims 5 base game:
-Focus on daily life.
Nothing supernatural, no crazy jobs like secret agent, no objects like a rocket or something, just focus on the life simulation first. Family life. And focus on objects commonly found in households, and commonly used furniture styles.
Could be some things for the wealthy like hot tubs, pools, a huge television and a piano perhaps, but they could save the millionaire lifestyle stuff for expansions.
-Have many regular careers covered.
How they handle them is another topic, but I'd want many common careers covered, the special stuff should be for expansions. 12 career tracks would be enough for the base game, they could be:
-Athlete. (could be a choice of three sports supported by objects in the base game: Basketball, Soccer and Football. More could be added later.)
-Have university in from the start.
I feel like they could do a lot more with this if it's in the game from the start, revamp the whole education system to improve it so that all expansions could later add to it. A sim's education could go from elementary, highschool, university.
The longer a sim goes to school, the more career tracks could be available to them. University would be optional. With a college degree they could start in all jobs and start some at a higher level, but university would allow sims to start any career higher up.
Initially there could be one university to study in, just a stereotypical university. But later expansions could add a magical university too, or a high tech one which could be required to learn things you can't learn at the regular university.
-Have cars, bikes and motorcycles from the start.
Sims 3 had it, seems a good thing to have in a base game, especially because how it interacts with the world. Need to be parking spots and all. And they could return cars with more interactivity like in sims 2. Be able to clean cars, get an alarm installed, teach teens to drive, etc. Several rusty old cars could be available really cheap which sims could fix up but can drive from the start. Initially it would not be that safe however, or they might not always start.
-Have both traits and sliders for creation.
All sims in sims 4 are pretty much the same. Three traits isn't much. I feel like a system of sliders and traits could complement each other well.
For example you can have a sexuality slider. A sim could be straight, gay and any shade of bisexual in between and this determines their interest in advances of other sims.
And on top of that there could be a trait that prevents a sim from having romantic interest in anyone, or makes them flirty.
Could also be a slider for how interested they are in technology. At zero they'd not use any electronic devices, at a bit higher they'd only use the minimum and prefer other forms of entertainment. In the middle they'd watch some tv. At the top you'd have sims addicted to social media, computers, etc. As it is all sims seem to be addicted to it.