These kinds of crossover products already exist. Theyāre not really for me.
I think there's probably a market in a slim book adapting the D&D Basic Rules to Narnia, with the basic campaign setting included. I don't think it's much of a money maker for Hasbro, though, since Narnia has been in decline for decades, based on its age, its overt religiosity and the bad faith people now attached to its estate. This is definitely a product to license, rather than to produce in-house.
Similarly, while I love the Princess Bride to a embarrassing-to-admit-in-public degree, its best days are also behind it. The license holders there have licensed everything under the sun for it (you could not imagine the number of card and board games based off of it). I think it was probably a mistake to license a Fudge version of the game, but that was presumably who showed up at the front door with a check that would clear. I do think a one-book version of the Princess Bride based off the Basic Rules would work, but again, I can't imagine there's that much of an audience for such things. (And, honestly, there are better systems for simulating swashbuckling action, unless some modifications are made to Basic D&D combat rules.) Again, a licensed product, not something Hasbro should be spending real manhours on.
Now, are there more popular contemporary fantasy franchises out there they could do this with? Probably, although I think the era of the everyone-knows-it fantasy franchise is probably over, with Game of Thrones and Harry Potter being the last two of that stature. And I think anyone who's got that kind of heat is more likely to strike a deal for a bespoke system like Game of Thrones or Harry Dresden or Rivers of London have gotten. But if, for some reason, someone like that showed up on Hasbro's doorstep, they shouldn't turn them away. But they also probably shouldn't spend a lot of time trying to strike those deals, and should just wait until they fall into their lap.