Honestly if they decided to go the route of "upgrading" the game I think that would be a great idea. There is never any reason for them to stop production of an iteration other then wanting to do major changes that require a new game or because the current game can't handle much more.
I agree with all the suggested by the OP but I'd also like to add.
-Open Neighborhoods, not full on open world but allowing us to travel between lots in the same hood without a loading screen would make exploring the hood more enjoyably.
-Rework Traits so they have more impact on sims autonomy
-Add a color wheel or allow us to play with the hue and saturation for hair and eyes.
