Do you customise worlds before playing?

Not necessarily before playing, though sometimes I can do that, but usually while playing. I don't know, I haven't started enough new worlds to say if I do one thing or the other. :P But I think yeah, I will always end up customizing the world one way or another, of course. I want to make it my own.

I'll add EP lots designed for the world I'm currently. I'm terrible at building so I generally download them from the exchange or use store lots. My biggest bane of exchange lots are people who use Sweet Treats in their build and people who don't give their lots an in-game or description. Especially if it had one on the exchange saying what it was and where it should go.

I'll also fix up traits, appearances and relationships of premade sims. The Goths get their terrible Tim Burton style clothing replaced with something more suitable for Sunset Valley's Addams Family, and have their traits switched to being closer to their TS1/TS2 personalities (including getting Cat Lover as a trait each). The Bachelors become the decadents, occultists and mystics they should be. And Thornton Wolff gets the Can't Stand Art trait because that bugs me so much.

If I'm writing a story, (which I usually am) I'll always customise the world to some extent or another - sometimes I strip them completely (and save the stripped version) and sometimes I'll just put several of my own builds in there. Or builds other people have made for me or for someone else.

I do modify the town sometimes. For instance, my most recent playthrough is set in a heavily modified version of Sunset Valley to make it easier to access content from the EPs. Here are the modifications:

• Removed the Showtime venues entirely. I dislike Showtime's careers, so they were useless.
  
• Moved the buildings added with Ambitions to more appropriate locations. Created a brand new lot on the beach behind the Langraab-Alto hill to use as a junkyard, and a new lot adjacent-to-the-left of the art gallery to place the consginment store.
  
• Added multiple high-rises to the three empty lots overlooking the oceanside from above a cliff, including the penthouse of a five-star celebrity, one containing an exclusive lounge, and the final one being a standard apartment.
  
• Added a dive bar and dance club to the beachfront.
  
• Created a miniature commercial zone outside of the town centre by including the salon from Ambitions, pet store from the Pets Limited Edition, and a church from the TS3 exchange to be used as a wedding venue. This involved moving one empty house to another lot.
  
• Added a horse ranch to the empty lot adjacent to the waterfall.
  
• Added the time portal to the large lake behind the small mountain for ease-of-access when I grow bored of the rest of the game's content and want to indulge in ITF's absurdly OP features.
  
• Added multiple houses I created over the years to the road of empty lots.
  

I ignored Showtime & Supernatural content because I dislike Showtime, and whilst I can appreciate the quality of Supernatural, it's not my cup of tea. I use ITF occasionally due to the superpowered Plumbots providing nonsensical fun.

Also, when the town became depopulated due to many pre-made Sims passing before they had children, I began adding households that I've made over the years to the town to bring it back to life.

I exclusively play in custom, community made large worlds that are already packed to the brim with lots of decorations/scenery, large lots, and CC - so more often than not, I don't see a need to or simply don't want to deal with worse in-game lag/freezing. But if I do customize a world in a save, it's always story-driven.

I usually remove the Showtime venues as I don’t really use those careers. I often add bars/clubs/lounges, as well as a martial arts lot so Sims can learn martial arts without having to pay to go to China. I’ll often remove the school lot, particularly if it’s the combined school-and-stadium rabbit hole, and put in Par Excellence Preparatory School. I noticed that Sims who are in the Professional Sports career lose their games if they go to the combined rabbit hole, so that’s one of the main reasons I separate the lots.
Since I got Into the Future, I’ll often plop down a Bot Emporium too.

Usually I remove one or two Lots (Both Showtime Lots) and replace it with either a Community Lot, a Golf Course, a Bowling Alley or something else. However, I keep the Performance Park and the Nightclub if I'm playing in Starlight Shores. Sometimes I build houses for my Sims to live in

I've been playing in the exact same copy of Moonlight Falls for years. I put way too much importance on visuals and the impression that the town doesn't exist solely for the sake of my current household (as it technically does). I couldn't play before stripping the town down to its bare bones, bulldozing lots along with households. I then meticulously created every single household from scratch, each with its own backstory; I would begin by making families and then splitting them up to move a sibling to one house, grandparents to another, etc. and then pretty much just mix and match. I'm an awful builder so I would download most of the lots, including the festival grounds, but then I would rebuild and redecorate to fit the identities living there. I'm pleased as punch with the end result and it brings me joy every single time I play, but it was an exhausting process.

I am NOT a builder, I make sims, play the game, or do one of the following:
-Download a build off the gallery
-Download a build off the gallery and make changes to it
-Just build a house and have it turn out ugly
the last one is the rarest

I mostly play the game as is. But I do add in extra Nightclubs and remove excess gyms and such from the game and replace them with residential houses. As for the townies, the only thing I really do is edit the Paparazzi. I remove the klepto trait and replace it with paranoid. I find it hilarious to see panicked paparazzi running around. They used to make me so mad but now they make me laugh every time I see them.