*Form 2 mythos decks: 1st deck, all cards that don't open gates in IH/DH; 2nd deck all mythos cards that open gates in IH/DH plus the Act cards
*Before drawing a mythos card, roll a die. On a 1-4, draw from the Arkham deck; on a 5-6, draw from the other deck.

Would that be a good variant? I think it could work quite well.

This isn't far off from a variant I've been using for some time. Warning: lots of technical detail!

First, background info.

When playing with just the Dunwich expansion, gates appear in Arkham 75.49% of the time whereas gates appear in Dunwich 24.51% of the time. This gives a ratio (Arkham:Dunwich) of 3.08:1.

When playing with just the Innsmouth expansion, gates appear in Arkham 64.71% of the time whereas gates appear in Innsmouth 32.35% of the time (doesn't add up to 100% due to double doomers in Innsmouth deck). This gives a ratio (Arkham:Innsmouth) of 2:1.

My goal was to match these ratios as best I can no matter what expansions are being used.

End of background.

The solution is to not mix in the Dunwich and/or Innsmouth mythos cards whenever I'm using those expansions. All other expansions in use have their mythos mixed in with the base game deck. End result: depending on what expansions are in play, there can be one, two, or even three mythos decks. Call these the base deck, expansion one deck, and (if present) expansion two deck.

When playing with just one of (Innsmouth or Dunwich): roll a die before drawing a mythos card. If you get 1-4: draw from the base deck. Otherwise draw from the expansion deck. (Mnemonic: On success, draw from expansion).

If playing with both Innsmouth and Dunwich: roll a die before drawing a mythos card. If you get 1-3: draw from the base deck. Otherwise roll again. If your second roll is 1-3, draw from expansion deck one, otherwise draw from expansion deck two. (Mnemonic: on blessed success, roll again to see which expansion deck to draw from).

Okay, now comes the analysis of how this should perform.

If using just Dunwich: gates appear in Arkham 76.85% of the time, gates appear in Dunwich 23.15% of the time (so ratio of 3.32:1). This is slightly off from desired, however if you add other (non Innsmouth) expansions you will get various double doomer, next acts, etc and the percentage of gates in Arkham will go down slightly bringing the ration back toward the 3:08:1.

If using just Innmsouth: gates appear in Arkham 66.67% of the time, gates appear in Innsmouth 30.56% of the time (so ratio is 2.18:1). This too is slightly off from desired; again if you add other (non Dunwich) expasions you will bring the percentage of gates in Arkham down and move the ratio closer to 2:1.

Next, what if using both Innsmouth and Dunwich? Then gates open in Arkahm 57.74% of the time, in Dunwich (expansion deck one) 17.36% of the time, and in Innsmouth (expansion deck two) 22.92% of the time. This gives ratios of (Arkham: Dunwich) of 3.32:1 and (Arkham:Innsmouth) of 2.52:1. Not perfect, but should be good enough.

Enough of analysis. In the real world I've been using this for the past six months or so with all sorts of mixes of expansions and have found that it perform as expected. While there are times when it seems I never draw from an expansion deck and other times when it seems all I do is draw from expansion decks, for the most part the rations match up with what was predicted.

Last note: this does nothing to address the dilution for Next Act cards for that I use Tibs rule (when resolving a card from any expansion other than King in Yellow: roll two dice. On a sum of three, flip over the top card from the three acts deck and resolve it immediately).

I used to do something almost exactly like that (you asked about it a month ago or so), but it had a few problems:

• Cards that aren't DH/IH/NAB will be seen less frequently and the others more frequently. The "Dark Man at Wizard's Hill" card is in the former deck and therefore will be less frequent.
• This variant doesn't scale when adding or removing expansions. What if you're using just DH or IH but not both? The math on the die would have to be re-done. What if another board expansion is released that opens gates? You'll have to compensate by increasing the chances of drawing from the DH/IH deck. Eventually, Arkham gates will be too rare.
• Two Mythos decks is a bit messy, and I prefer not having to roll to draw one.

But I guess it can't hurt overall. It's just not for me anymore.

Your "what ifs" don't concern me as I always play with everything.

Btw I'm aware of your extra monsters per expansion towns variant, but from my experience what happens is one of the three:
1. No gate opens at expansion towns. At all. Your variant doesn't fix this.
2. Either Dunwich or Innsmouth becomes impossibly active, but never both. When one is already active, your variant doesn't add much.
3. Around 2 gates total open in expansion towns. In that case, your variant will not change much as one extra monster won't make a world of difference.
That's why I don't like it. I will give my own variant a few test plays and come back with results.

1. No, it doesn't fix this.
2. The infrequency of gates is countered by a burst in monsters when there is activity. So on average they're about as active, but there's a huge standard deviation, yes.
3. Once in a while yeah you'll get more, but the extra monsters work well enough to make sure that the DH is in threat of opening.

Different strokes for different folks. I didn't like how Arkham activity started to dwindle, and all those things I explained earlier.

Fair enough.

Tibs said:

Regarding not scaling for adding/removing expansions and needing to adjust dice: I assume you are referring to OP variant. The version I detailed works no matter what expansions you use as long as you keep the decks separated as noted (one for mythos that came with Innsmouth, one for mythos that came with Dunwich, one for everything else). You do have a valid point regarding needing to rework if there is another board expansion that has unstable locations - then it's time to crunch the math again.

Regarding two decks being messy - yup, they are. Even worse when there are three. OTOH, it works for me for now.