If anyone's played Space Hulk, you can use that game as a good example of how you might expand this game to accomodate 5-6 heroes. If I were designing a scenario to be used with that many people, I would do something like this: Split the heroes into two groups of three, or two and three, or whatever. Each group has its own starting location, and outside of the starting location is an adventure/monsters that are tuned to 2/3 people. Each group has an objective to complete in its area, and once it does, it can move on to a central area (one or more of the very large tiles) which has challenges that are more in line with 5-6 players. Afterwards, there can be a single final objective or exit, or there can be two again (which gives the heroes a change to rearrange their teams.)
You can scale things up in one of two ways. If you've got the models or don't mind proxying, you could simply continue the normal progression - so that shadow dragons would have two masters with 5 players, two masters and a minion with 6. Or, you could max monster groups out at the 4-player levels, but allow more monster groups than normal. This would be harder to balance without playtesting, I think, because there's no set idea of how many monsters at a time is an appropriate challenge... plus, more monsters get in each other's way, even in the biggest rooms, so there's diminishing returns.
I have no idea if you could build a board like this, even with all the expansions, but it would be really cool to make an "octopus" 8-player board, where every hero had his own path to start out, but after facing a minor challenge the path would merge with another hero's, then the groups of 2 would merge, and finally all eight heroes would be fighting for the exit.
Possibly against eight lieutenants :-)