I did post something like this but it didn't work for some reason.
Oh, well.

Setting Up
I would suggest making a new game load. You don't want to play this challenge in a world that you are currently playing with. And putting your life span on long.
In CAS, make a couple. One of this sims will be your main character. This sim must have the Mean and Evil traits and the Public Enemy aspiration. The other trait can be chosen by you. The other sim must have the Family-Oriented and Loner traits. Their other trait and aspiration can be chosen by you.
Move them into any lot, pre-made or built by you. The only requirement for the house is that it must have a basement. The basement must have at least one bathroom, 10 beds, fridge ( Not a stove, the fridge is so they can eat food you have made) and a seating area of some sort. You may use all build/buy mode cheat to furnish the house but no cheats after that.

Goal

You goal is to sucessfully kidnap 25 sims, without your spouse finding out. If they find out, you have to options. Option one is to divorce them and move them out within 24 Sim-hours. Option 2 is to use them to kidnap other sims. But if you choose option 2, you must kidnap 30 sims.
Of every kidnapped sim, you must make them stay alive. If one of your kidnapped sims died you replace them with 2 more sims.
I suggest making a sort of airlock room with only one door, then moving the door when needed.

And some rules (Yay)

You must keep your kipnapped sims alive.
You can't move any of your kidnapped sims into your household, but your sims can have babies.
Your main sim can't get a job, but your spouse can.
If divorce your spouse, you must get married again before you can kidnap anymore sims.
Post pictures. ( Okay, you don't HAVE to, but I like looking at them.)
Have Fun!!!

If you have suggestions please tell me, if you think I should revise the rules please tell me!!
Enjoy!

But there are no basements yet in the Sims 4?

Basements were part of one of the free updates. Just before the go to work exp. pack come out.

It came in the free update just before the GTW Exp Pack came out. If you can update for some reason, feel free to use an attic.

Ah, gotcha. Was unaware of that. Good to know. This challenge sounds right up my alley. It might be more fun to change it up a bit from murdering and torturing random sims...

You can use Scumbumbo's "door locks" mod rather than moving the door around. Here the direct URLfor that mod: http://modthesims.info/d/552565. Be sure to get version 3 because it fixes some bugs in prior versions of the mod and also adds the ability to lock door by gender.

i love this, gonna try it out tonight!

lol i actually did this but my sim was single. I had the whole world in my house i used 2 main mods to help with it.
the door locks mod already mentioned, and "invite to home indefinably" http://modthesims.info/d/545229
This mod makes the sims in your house use things like it's their own house. they sleep shower and all that without issue. To trap them i'd have what i called "the waiting room. A little shed with a couch in it and as people would walk by i'd chat with them and click "sit together" The basement door was in my shed, so as i locked the shed entrance I'd open the basement. All went pretty smoothly til i hit about 35 sims. Weird glitchy craziness happens around there. Sims all just stand around frozen, some lose animation, Not recommended you have more then 30 in one place. =)

Lol. I managed to get 38 sims, and I went to work and when I went into the basement to check on them they spontaneously disappeared.

Oooooh, really interesting. I'll give this a shot and maybe upload pictures later :P

EDIT: For some reason when I have people in my basement they say the "Okay I need to go somewhere blah blah" and they just... POOF! Disappear! How does one fix or prevent this?

I hate Sims 4 for personal reasons so I'll do this in Sims 3! It's gonna be FUN!