So house rules are one thing, but my fellow Arkham players and I wanted more. I am sharing my own campaign formula for those of you who have a solid group of friends and all of the expansions and simply want more.
We are currently halfway through this campaign, and we have lost 23 investigators, but vanquished at least half of the ancient ones. With one notable exception in which we were not victorious. This is going to be something of a long post so the short version is that we have combined all of the expansions and come up with a gameplay mod involving a constant revolving team of 6 investigators. (Always six to achieve a permanent 4 man rule set)
I'll try to break down the collective rules into sections, feel free to ask about specific things as some of these house rules are so intrinsic to the way we play I may forget things.
1. Ancient Ones.
So we decided early on that only two ancient ones would be capable of completely destroying Arkham if they won. This was not based on mythos, but on the mechanics of the Ancient ones. So we naturally looked to Shudde Mell and Azathoth. Keeping this in mind, we carefully devised an order we thought best in terms of increasing difficulty with Azathoth at the end, and Cthulhu in the middle. Other than the order, there are additional changes related to victory or defeat against these ancient ones that will be detailed in other sections. Combat against them is always based on the number of investigators who are alive when the players have resolved the "Start of Battle" portion of the Ancient one sheet. So max of 4, min of 1. In all other ways combat against the Ancient ones is unchanged and makes use of the Battle Deck and standard rules.
We decide these with dice roles. A cursed die is rolled first for Guardians. On success, the three guardians are shuffled, and placed in a line. 1-2 for the first, 3-4 for the second, and so on. For institutions, a Blessed die is rolled. On success, the rules are the same as those for guardians. Then for heralds, we actually use a 12 sided die, shuffling and placing them in a line. This ensures that as the campaign progresses, there is steadily less and less of a chance to draw a herald. If either Father or Mother are rolled, a blessed die is tossed to see if the companion herald will join it. Other Herald mechanics include defeat of the actual Herald. Any Herald that can be defeated as either a monster or ancient one is removed from the campaign completely if defeated in a game. In addition, no herald can be used with its matching Ancient one. (No King in the Yellow herald + Hastur)
First we separated the investigator sheets according to expansion, then split them into decks of 8. Once we had done that, we ordered each individual deck of 8 from best to worst, with worst on top, then placed the piles in a line. Drawing one from each in order to form a stack which runs ROUGHLY from worst to best. How this order is determined is completely up to you, but this is the method we used. Once this is done, we pulled ALL of their UNIQUE possessions from the item decks, and placed them with the sheets. We actually back a box dedicated to JUST the investigators in waiting. This decision about the unique items will make sense shortly. Randoms are not dealt until the Investigator actually enters the game. Oh and no choosing who you get initially. Roll dice, much fairer and leads to less meta-gaming.
3. Ally Deck
All allies are used. This will make more sense after the next section.
This is a mechanic which ensures a continually more brutal experience as the campaign progresses. During the game, any items discarded through use, or from loss (death, stolen, encounter which causes a loss) is put in a discard pile for after the game. After the game one common, unique, spell, exhibit, ally, and skill are all discarded permenantly for the rest of the campaign. The items are drawn randomly from discarded items first, or from the tops of the decks if none of that type are available.
Exceptions to this general rule are as follows.
Every time an Ancient one is Victorious (and it doesn't cause the end of the campaign) One additional item is discarded, i,e. We lost to Atlach, so then in addition to one from every deck, we lost an additional spell every game. We decided that the order of losses would go : Spell, Unique, Common, Skill, Exhibit, Ally.
Beyond that, you'd probably be out of investigators anyway. In addition to this, Every item discarded in the battle against Cthulhu is discarded to box right away, and before the battle with Azathoth, the remaining decks are randomly cut in half with half being discarded before the fight even starts. This ensures that the fights against these two opponents are especially fierce.
4. Inventory and Carry over between Games
We decided on a 5 item limit for characters. Meaning, 5 of each type of item. This allows (theoretically) for 20 items, 5 allies, and 5 skills. This may sound broken on paper but it works surprisingly well in practice. Between games EVERYTHING carries over except for trophies, and clues. At the beginning of the next game, surviving investigators get one dollar and the clue tokens listed on their sheet, and the hell simply continues.
Now when I say everything carries over, part of what makes this balanced when combined with the attrition mechanic should be obvious, getting knocked out, or going insane now has real dire consequences. Losing items puts them at risk to be lost for the entire campaign, and picking up an injury or madness is now something that sticks with you for a long time. Corruption cards can be funny in one game, and the death of you in the next, and so on.
5. Spell Mechanic
Due to the notable difficulty increase of the campaign mechanic and because frankly, I didn't like the base game spell mechanic, we came up with the solution of having combat spells stay on you as a buff for the rest of that combat (this would include multiple monsters in an area or the entire battle against an ancient one)
You can have up to two hands worth of spells as a buff at any given moment, and in subsequent turns of combat you are perfectly capable of adding to this with weapons. This is another mechanic that may sound broken, but works extremely well in practice.
6. Other House rules
All monsters are face down until combat is initiated, and all gate markers are face down till someone goes into them. since I usually play a sort of Arkham master role, I do the work of green monster movement. In the case of escaping other worlds, even if there is an open gate, if it is face down, it might as well not exist.
I'm sure there is something I'm forgetting, but that's the gist.