I'm afraid I never saw much about The Sims 3 and I'm not veteran player. I just know some things due to watching my eldest brother play The Sims 2 on occasion when little.
CARS:
I've read of people having so much trouble with the Sims 3 open world and considering there at players even now who can only do so much via their computer and internet... I think the loading screens are fine enough. So maybe do cars like how they were done in The Sims 2. It will still have the loading screen and the separated worlds. It would need a short additional animation entering and leaving the car as well as for the car pulling up or heading out. (Maybe even add in the school bus animation for kids to teens.) Maybe even call up taxi drivers.
Extra: Be able to build garages, buy new cars or repair/make a car from parts. (I think that would be freaking awesome.)
BUILD MODE:
Yeah, definitely spiral stairs. You can sort of make them in similarity to a stairwell, but the room required and the space it takes up is evil.
I'd agree to some extent on the larger lot sizes, but if this is base game only... I think we'd need a few more places added onto the current maps for Willow Creek, Oasis Springs and Newcrest... *Tilts head...*
Extra: Vending games for the base game. Get to Work and University have them and if you have some of the other packs you get the food stalls. They could be so useful when visiting a park or other commercial lot.
LOTS:
A Graveyard lot with some already preexisting tombstones with name, death type and/or epithet like how the decorative one in Pets has, but deceased Sims in your game will actually be placed at the cemetery… none of that, “You have to be on the lot the Sim died to collect the urn or tombstone.”
The last three things don't have to do with base game and would extend into expansion packs. *Shrugs*
I'd also like to see some old NPCS.
NPCs:
From the The Sims I’d like to see
- Social Bunny for those who have no time to socialize or if their socials are low.
- Imaginary Therapist for the times a Sim sees or experiences something traumatizing enough to cause a meltdown. (I liked the meltdown and hallucination aspect... I mean, I totally get that and so would a lot of people these days. Therapists, psychologists and psychiatrists are freaking normal these days! It wouldn't have to have anything to do with the "erratic behavior" trait BS.)
- Burglars that may actually pick a lock even if you set a “Lock doors” option – your Sim can be woken up to scare the burglar away (fight burglar option?) like how Sims take care of fires on their own. (Since this speaks of base game and not with expansions.)