If you lost interest in the game, what did you do to develop interest in it again?

Probably an unhelpful answer; I don't do anything. I don't get much time off, so my recreational time is exclusively for things I'm interested in doing. I haven't really played the game to any significant extent since November, but I'm sure the desire to will come eventually. Until then, I won't touch it :p

The only thing keeping me going is new content.
But my situation is a bit different because I am deliberately holding back on the progression of my sims until I get University.

I still haven't finished Strangerville yet, but what had me playing it was adding Mulder, Scully, and aliens into the story line.
I was having fun with that until I was distracted by reformatting rewriting one of my stories.

@CK213 I feel the same way about University, so when I do play I have aging off. I really hope this is one that they will be adding next, or really soon.

Try to collect everything (the crystals, frogs, fossils, post cards, etc...) and make a retail store/stores to sell it all.

Since you play with aging off, make a household that completes every branch of every career, then make a retail store with them that sells all of the items unlocked from careers. (So all your other sims can buy it without cheats)

Make a garden with a perfect of everything.

Try to get a gold of every magical totem from JA.

Make a sim who wants to be a witch. Of course witch powers aren’t in the game yet, but there is voodoo dolls, potions, relics, mischief, weather machine, etc. make a sim who pretends to be a witch by using all that stuff and dressing and decorating their home as gothic as possible.

Make restaurants with different menus and styles. I just made a Mexican restaurant with ceviche, burritos and taquitos from the food stalls, some items from JA, etc. It might be fun to create a sim that looks like a maffia boss and have them open an Italian restaurant.

Make public lots where your sims can work on skills and complete aspiration goals like an art center to play music and paint, or a science center to work on/launch rocket ships.

Just a few things I do.

A good backstory, I like to start with some kind of basis to play my games. For example, in my current game, I started with 3 separate households.
So, for example, one household is as young war widow with a child. With the help of her parents and in- laws, she went to school while raising her son till he was old enough to start school. Hoping first a new start, she moved to Oasis Springs with her son to start a new job as a political activist. She is from a rural area and has traditional values.
Currently, she has just married Johnny Zest and her brother-in-law, just aged up from a teen, moved into Johnny's old trailer to become an astronaut, there is plenty of space for a rocket ship there.
I can't wait to see what's next!

I've started with doing a challenge and then I was confronted with a new way of playing that i now do without the challenge anymore. I used to play without an objective to my story so I ended up doing the same stuff over and over, using the same objects, the traits that I liked most... And I used to use a lot speed 3. I'ld put all orders in list and then hit speed up. Mainly because of doing the same range of stuff and knowing already how sims reacted. The challenge made me use other things in the game and use a slower pace, as I had to think about my objective. I started using more speed 1,taking screenshots, enjoying more each detail. That has made me fall in love with the game again.
But I also think that when we need a time out, then we really should be away for some time. If it'll be like me than the will to play will come again. And when it does, then maybe you should try a new approach.

Way ive maintained interest been first no cheats of any kind and going tru most of the packs as they come, i also tend to focus on milestones collectibles and such to keep me entertain since i don't use cheats, alot of work goes into making my Sims efficient at certain thing's.

I play a very large rotation a lot of different families. Sometimes I will take a break for a few days and think about what I want to do about my families/ roommates. Do you have a painter that you send around all of the towns painting pictures that he can sell? the sketch pad probably makes it even easier now. Also a photographer can do the same thing I cheated and gave my photographer a yacht he lives on ( downloaded from the gallery), it is by the water in Newcrest. And, he owns a camera shop in Oasis Springs, also downloaded from the gallery. It can be fun looking for spots to paint or to take photographs. I have one sim that makes periodic trips to the ruin pool in Windenburg to see if she can find Emily to take a picture of.... hasn’t found her yet. How about creating some interesting and different clubs? My criminals have a club where they meet at the solar lounge and play cards and drink, one has a jealous wife, whenever he goes anywhere she starts getting panic attacks.... lol. One is a single father.... that shouts forbidden words, I have a gypsy family with lots of kids that live in a caravan with a big tent under it. The Caravan only has one bedroom where the parents and baby sleep, one bathroom. And a couple of bushes. They live off the land ( no jobs) which has gotten more difficult because not all of their plants produce each season now. The Father and teenage son fish, they live in that lot in Windenburg with that little pond with a little wooden pier. The mother paints and always seems to be cooking. They have a lot of that movie stuff Bohemian furniture. The whole family goes off to dig and collect what they can to pay bills. I bought the father a work bench, so he can start to make things to sell. The teenage son is now going through a phase where he doesn’t want to be around the family.... lol. I love the active careers. I have gotten a scientist, detective and doctor to level 10 now. My collector sim still has many things left to find... there are still several insects she hasn’t found. And, I haven’t gotten into the real jungle yet. Just do something different, something that makes you laugh..... the parenting pack makes me laugh.... I still have problems trying to control children to come out like I want. I have aging off most of the time in my main game. I just have fun with the game and I don’t use custom content or mods.

I tear off some builds and get my Sims some new from the gallery.
Change outfits in CAS, make some new Holidays, go on a vacation, change careers... This mostly helps.

Honestly? Every time I get bored and uninterested in The Sims, I just stop playing for several months... or even a year... until a few more packs have been released. Then I get back into it again.