Hi all.
I recently found another Arkham group (My old one mostly graduated 2 years ago before me).
The new group say that they too have the lack of players problem so they were glad to find someone else who likes it.
They also say that they always have all the expansions mixed together but are aware of the dilution problem and have tried to figure out a fix but because they don't play it much they haven't had a chance to play test it.
Hence I'll share what they are thinking of doing.
I think they proposed making a mythos deck at the beginning containing 1 mythos card from each expansion and some number of base arkham cards. Then whenever a mythos card from a particular expansion is drawn they add 2 more cards from that expansion to the "created" mythos deck. That way they increase the chance of a particular expansion "doing something" .
In this way each game it's random which expansion gets center stage each game but all expansions have a presence.
Now I haven't tried this and I'm interested if there's a general way to handle the dilution issue that's generally agreed upon.