"Modular playsets" have been done, and done really well, but just not by Hasbro... I think the problem with them is they're cool to the 30+ crowd, but to a 12 and under, do stackable hallways matter?
There was a playset put out eons ago for military toys. It was the first time I'd seen anything like it, and it's nothing you can buy now (short window of availability). Tiny little cheapo toy company in China made it. It was simply AWESOME! I got mine from someone who collected 1/18 military figures, and that was how I actually heard about this thing at all. I wish I'd bought a dozen of them at the time but alas, hindsight is always 20/20.
It is very customizable, very simple... Walls slot into posts which plug into floor squares. Everything is perfectly square, and can be built atop/under each other, and side-to-side, front-to-back, etc. Walls are thin printed sheets more or less (cheap in feel and look). It makes a cool display for really military stuff, Star Wars stuff... almost anything. I loved it though. It's not amazing, but it's fun and looks neat with figures around it.
That said, I think Hasbro looks at that really for what it is... It's like a stand on steroids for older people. A cool way to display toys rather than play with them. And while I personally want that too, do kids? I honestly feel I would've at a younger age, actually. I loved the Terror Drome playset, and it was the action in the little hallways underneath that you could open and play in that I enjoyed the most as a kid. But I was a weird kid (and adult), so do kids today want that or do they want a playset with all the bells/whistles that make up something like we're accustomed to seeing from Hasbro? I think they'd lean that way with one, not in the stackable display route. I mean, the TFA Falcon kind of sums it up. But hey, any year anything is possible at this point too. Hasbro has every year to try something else out, if they want.
I have seen some cool things in the world of customizable displays or "playsets" though, but I just don't think Hasbro's going to bite on something like that, as a big Christmas time toy/gift idea.
And looking at what garage casters have done, it's doable on the small scale, but it ain't cheap for the buyer.
Also a risk of dealing with someone who just decides life's in the way and they can't get it done... even if they've been paid.