So Karl canonically is not human and has a non human form. Now what I am suggesting is that Karl gets lost in the time travel sauce a little too much and sort of gets stuck like. In between the timelines? Idk if that makes sense but hear me out;
So Karl gets stuck. He physically cannot travel through time anymore and he is stuck in his weird inflatable tube man form and he has. No idea what to do so, panicked, he tries to open a portal somewhere... and it works!
Aaaaand someone just fell in!
Now at first I was thinking Tommy could be the one to fall through, but that can be changed. But hey, some people simply have main character syndrome so for the sake of explaining this au, I'll just say Tommy gets sapped into the portal.
So him and Karl are both in this in-between timeline (also idk from which era this Tommy is btw, that can be up to whoever else) and Karl gives Tommy the book he used to time travel and is like "help ;-;"
So the whole AU is Tommy travelling through various timelines, trying to find a way to not only help Karl but also stop all the bad things from happening.
Meanwhile, for angst purposes, Karl is still trapped alone, with no way to talk to Tommy until he comes back to that in between (where Karl opens portals for him and such) and still, Karl is losing bits of himself and his memories, unable to write them down and with a constant flow of timelines and possibilities constantly flashing before his eyes.
Maybe one day Tommy comes back to the in between and Karl is barely coherent, barely solid and barely remembers who he even is... - 🐲
Tommy is now a time traveler and trying to help Karl. Man this is good. We have angst potential and some bonding potential between Karl and Tommy :]
Just the angst we can make for this au, especially with Karl losing bits of himself.