Or you just generate overlays.
An overlay that is covering an entire quarter of the Hero Quest Board: That leaves enough freedom to play with larger rooms, Hallways in odd places or odd sizes, labyrinth structures.
You can have some which would cover also the adjacent Hallways to allow for more design freedom, and some that leave the adjacent Hallways free.
And it allows for reuse with the original Board. You can sell it in two modes:
1. With rubber underside for shift-free modularity,
2. and as digital copy like Heroicmaps, as High Quality PDF: then everybody can print them fitting to their Board Size / Material.
The argument could go: "You can do that with modular single room tiles on the original Board, too".
Sure. But if you surpass a critical mass of modular cardboard tiles, at some time, it will start to shift around, losing coherency.
I recommend looking at the Forum Board/tile section, for "Prison Block"; that shows the principle pretty good.
Another Idea, if you want to go into production:
Modular Tile fixing door Counterbases. Some kind of clamp that would clamp modular tiles so they don't shift, but other than the plastic tiles you find everywhere, don't damage your cardboard because they'd have a soft inlay. They'd have a as-thin-as-possible underside so they don't lift the board too much, and a flat upper side that serves as base to place a door on it. With a door placed on top, they are more or less invisible.
I wouldn't do them, but if someone picks this idea, i'd spend some bucks on it. This my idea is under the lesser gnu license - you may reuse it to make money with it, but you're not allowed to reserve it for you alone.