I hesitate to say this, as my Arkham Horror storage solution is absolutely busting at the seams with all 6 expansions shoehorned in there, but am I the only one who would like to see an expansion that bolsters the existing locales and mechanics?
For instance, Arkham encounters are up to ... what? ... 35 cards with all 6 expansions in the mix? Dunwich, Kingsport and Innsmouth are all languishing at a "mere" 14 encounter cards a piece. I'd love to see the number of expansion location encounters brought up to par with Arkham, or at least in the same neighborhood (HA!). I'm the kind of player who will gladly sacrifice theme for variance (though I do enjoy the occasional stripped down theme game); I mix it all in and practice good Mythos/Gate deck management to ensure the expansion boards see action. These new encounters could make use of all the special rules introduced by later expansions: monsters surprising investigators, being barred from locations, conditions, detriments/benefits, etc. More importantly, I'd love to see certain "fixes". For instance, a new Cult encounter deck would be absolutely awesome - with encounters that actually make joining the Cult worth it. Sure, you become corrupt, but there should be benefits, a reason to spend all those trophies to gain membership. I'm sure there are other aspects of the game that could use a goose. Reissued Arkham GOOs, bring them up to speed with their more powerful and frightening expansion brothers and sisters. Drawing Cthulhu should send a chill down your spine. As it is, compared to some of the more recent Ancient Ones, he's a schoolgirl.
Of course, this seems like an extremely niche product. Only those who own all or most of the expansions would consider it a must-buy.
That said ... you'd get my money!