I think the game would work well with 5 players and would love an expansion with that. Beyond the obvious, I think another key and backpack might be necessary.
A bigger board with more artifacts would of course be nice, but hard to do.
I definitely agree with the key, the backpack, however, would only be worth adding (imo) if there were more artefacts added (perhaps 12 and/or 18 points). This would only be possible with board alteration though and would have to be done carefully.
as well as a modification of the amount of cubes drawn during dragon attacks, since the bag would become more diluted. The attacks would be more frequent though, so maybe that's unnecessary.
Actually with more players you'll get hit more than in a lower player count game since you have say 1/5th of the player cubes in the bag but the dragon attacks up to 5 times between your turns so that cancels out. But the more dragon attacks there are the more black dragon cubes get taken out and so more attacks will lead to more player cubes being drawn. So when you increase the number of players you will need to reduce the number of cubes drawn, perhaps adding in an extra 2 and extra 3 stage to the track. Or alternatively adding more black cubes at the start (say 30). Of course this doesn't really matter - it will just shift the feel of the game to a more dangerous one if you don't account for it.