How do y'all play Sims 4

It depends on what I want to do with each save file.

On my PC version, which only has the base game, and I couple packs. For some, I have one main family, which will branch out when their kids move out, so it is technically a legacy family with everyone related to the main family. I also have some themed families, for example, I am on a Legend of Korra kick and Avatar in general, because I recently re-watched the series. With those, I have found Sims based on the characters, and moved different versions in the same save file, and have them all interact with each other. So, think Alternate Universe versions altogether. Other families are random, some may be attached to a challenge, or just made to get away from my other game play.

For my PS4 version, I pretty much have all the packs. Supernatural stuff, and normal stuff all make up my game. I have my Supernatural Sims and normal Sims living together in most save files. 

Like my PC version, I will sometimes theme a save file to a favorite series, like Frozen, Harry Potter, Avatar the last Airbender/Legend of Korra, etc. I will either make my own, or download the ones I like from the gallery, and move them into the same save file. So, canon character set ups will meet AUs. For Harry Potter it would be him as an orphan, but also a version of him raised throughout his life by his parents, and both versions getting to meet each other. 

Also, I keep some save files for just Sims I don't base on anything, or do a mixture of Sims I create interacting with Sims from smaller series I like. 

I feel that switching things up in general makes me a bit more invested in my Sims, and makes game play fun for me.