Warning: contains spoilers for Fantastic Four #31!
In the latest issue of Marvel Comics' Fantastic Four, Mr. Fantastic and the Thing went on what may been the First Family's darkest mission ever, and they might still be on it. In Fantastic Four #31, Reed Richards attempts to spend some quality time with Ben Grimm by adventuring to a brand new dimension and exploring the unknown together, just for the fun of it. However, it all goes south pretty quickly due to the nature of the dimension itself. Thankfully, Mr. Fantastic and the Thing find a way to escape... or do they?
In recent issues of Fantastic Four, best friends Reed and Ben have been struggling with what they were shown by the villainous Griever. Within her necro-spheres, it was revealed that Ben will most likely kill Mr. Fantastic, based upon some unknown misdeed on Reed's part that has yet to happen. Then, Ben was taken over by a symbiote in Marvel's King in Black event, causing Ben to unleash his darkest thoughts about Reed. As such, Reed takes it upon himself in Fantastic Four #31 - from writer Dan Slott and artist R.B Silva - to create an opportunity for them to spend some time together after all the tension they've been feeling.
While the Thing would've been just fine with fishing, Reed has other ideas and the two of them use the Forever Gate to visit a brand new dimension to explore. They soon realize that the entire realm is a thought space, allowing whatever they can think of to come into being. As such, Reed starts seeing the answers to every mystery of the Marvel Universe. However, Ben starts hoping for some giant monsters to fight, putting the heroes in a very dangerous situation. Unfortunately, while Reed had ensured the Forever Gate had the energy to return them home, other members of their household used the Gate in the interim, effectively trapping Reed and Ben. However, Reed soon came up with the answer - they simply thought about how much they wanted the Forever Gate, passing through its Thought Space replica to return home. Once there, Reed jokes they may still be in Thought Space, experiencing a fake world patterned on their expectations. The two laugh off this idea, despite having no way to test if this is the case.
Reed was able to construct the Forever Gate because his genius allowed him to fully understand its construction and functions, but that same genius could make Thought Space as convincing as it needed to be, especially if the dimension works on principles of illusion rather than creation (the comic suggests this is the case, since Reed gives up on the answers to the universe's "mysteries" once he realizes he's just seeing what he wants to.) While Thing jokes that Reed wouldn't have wanted to return to a frustrated Invisible Woman, it's exactly this argument that puts the two at ease, meaning that it's actually a necessary part of their desire to return home.
It's doubtful that Mr. Fantastic and the Thing are still trapped - their appearance in other titles should settle the matter for readers, though it will now always be possible the entire Marvel line is just Ben and Reed's Thought Space fantasy - but it does create an interesting reset point for future writers. As an example, Dan Slott could now go as big as he wants with future narratives (like Ben making the decision to kill Reed), then have Reed and Ben rescued from this thought space by the rest of the Fantastic Four, with the explanation being that they had just been imagining it all. But the far darker idea is that Mr. Fantastic and the Thing will never truly know, beyond a shadow of a doubt, that the world around them is real. Perhaps that's part of the cost of being in the Fantastic Four, but for two characters who set so much store in their families, it should be an existential nightmare... if they ever get around to considering it.