For my custom template I modified almost all premades, some were deleted. Most modifications were clothes (especially hot/cold outfits), accessories, make-up, hairstyles, etc. A few had their physical characteristics modified. Traits/aspirations/careers unchanged.
I also modified a few lots; parks, gym in Windenburg, bar in Sulani (included a bowling lane), bar in Glimmerbrook, the camping lot in OR and put my Itty Bitty Bunker build on the RV lot in Strangerville. Plus, I deleted the retail lots in Magnolia Promenade and changed them to residential. The only 'extra' venue included anywhere is an Italian restaurant in Newcrest. I add other venues on an as needed basis.
Lastly, I put in a copy of favorite Sims from other save files as not-in-world townies along with modified game generated; service Sims, vampires, townies, and a mermaid. I use this template to begin all new save files. It gets updated on occasion, especially when new packs are released.
It's a simple template, not a complete overhaul of everything as some have undertaken and achieved. I'm in awe of y'all that have taken the time to do that! What I've created works for me and my playstyle, so I'm happy. Especially since I get to see my favorite Sims in every save. :)
Happy Simming! <3