For me, redoing Sunset Valley means getting rid of things that slow down sims down, like stairs that make you go up, up, up another floor to just get in the front door.
A couple places I always swap out with my own builds. The first is the library – that almost always drop sims at the side and make them route back to the front and up stairs, as well as having another floor inside, making it impossible to see everything and everyone at a glance…

I do keep all the bookcases, so nothing is lost in transition, and the main rooms keep their same layout; but some distracting toys are moved outside.

And, of course, the gym, which again takes sims upstairs to get in and then up more steps to work out. So, for the gym, I keep the first set of stairs, which aren’t outrageous, but I move the upstairs stuff down to the main floor. So, the upstairs t.v. is now in the lobby, the upstairs bath is in a far corner, and the pool isn’t as big as it was; but it still has everything it did, minus joga rooms.
The art museum is condensed too, with stairs on either side and all art work on display on that second floor.

Some lots really only need simple fixes to help routing like deleting the stairs in the central park and front fencing that can trap sims and animals.

Others like city hall and the school mean using cheats to flatten the terrain after deleting the stairs. (I use constrainfloorelevation false and drag from one end of the lot to the other back far enough to cover the building. Then I turn off that cheat and go around grabbing everything that is floating (like the building) and releasing so they drop back into place. (If they don’t drop where they should, or at all; I simply click back and forth between buy and build mode.)

I redo some of the housing as well. double beds against the wall on one side didn’t make sense to me. So some sections that were three wide with one window became four wide with two windows.
Some of the lots get an extensive overhaul, but I try to remain true to the original look and layout as much as possible. For example, the Wolf mansion had only a gym and bathrooms on the ground floor and bedrooms on a third floor. All that was condensed to one floor on a normal foundation, but it retains the same basic layout.
The Sekemoto home with it’s in-law suite in back gets a mud room added in back and a spare room added in front on top of old unused porch space. After all, a family of five kids is desired, so you need at least one more bedroom, right?!
I love redoing homes and lots, whether sims live there or not; and I move in the bin families as well. So, some of the cheaper, smaller lots get a bit of a makeover so the bin family can both afford to buy and then have what they need to live there. For instance, this home needed another bedroom to fit the Lum family, yet they didn’t have much money, so it’s still small, but it fits a family of three.

Any way, that’s a look at how I set up a world for initial play. It’s minimal compared to some and overkill for others. That’s yet another reason to love this game! We all get to play it our way! Happy simming!