I love Arkham, as does my usual groups of 5 a we have played it 5 times and loved every one, some vanilla, some with Dunwich Horror. I have a 7 possibly 8 player game coming up which everyone is excited for (only 1 new player), and will play with Dunwich and Kingsport.I don't think that you'll need anything to make it a challenge for 7/8 players! I used to play almost exclusively with 7P and I can assure you that the threat of the Gate Limit along with the risk of "A Gate and a Monster Appear" will keep you worried. Similarly, the previously mentioned 2 Monsters per Gate can be hard (especially if they are both Yellow or Green). And then there's Monster Surges. You will not enjoy Monster Surges. But deffo use a big box expansion - having the extra board to keep all the Investigators busy.
What are recommendations on house rules or variants to give enough challenge and make it fun? I will be in GM role, and I was thinking 2 monsters per mythos card.