I did a few posts about this a bit ago actually:
Karl
I imagine Karl not having a stationary limbo. What I mean is that his limbo is that he can visit other people’s limbos. He can only go to the limbos of the people he met time travelling though, not anyone from his timeline. He can only stay in certain limbos for a set period of time, though he can always go back later. He can even temporarily create small places for all the people from one tftsmp time period to all hang out together, however those don’t stay when he leaves.
Quackity
I imagine Quackity’s limbo to show him what he brought upon himself. His limbo looks just like a casino, it’s filled with people. All of the people in the casino look like people from the dream smp. Loads of people are losing money and Quackity is getting it all. He’s rich! However, whenever he tries to talk to someone they ignore him, no matter what their relationship was while he was alive. The only people that respond are the Sapnap and Karl lookalikes. But, because Quackity left the real Karl and Sapnap, they are always mad at Q and argue with him. If the real DSMP members that are in the afterlife try to talk to him, he ignores them because he believes that they are illusions.
