For Starters, ELDRITCH HORROR is probably my favorite game of all times. I'm a little bit biased on some of the new stuff in MOUNTAINS OF MADNESS, though, for example Preludes or new Sideboards I don't care about. Usually I just take the new stuff to pimp my vanilla-game, without having to change too much.

I had this idea for a completely optional small-box Expansion which would only add some cards to existing decks, with the goal of making the game harder and more challenging: these cards would include Tests, which would require at least 2 Successes on rolls in order to be counted as a success. Or give you different results, whether you have rolled 1 or 2 successes: for example, when resolving a "failed"-result on a complex encounter 1 success would be enough to get you alive without gaining anything, while 2 successes would still get you a consolation prize.

There shouldn't have to be a lot of them, just a couple to make a little bit more scared. Maybe 2 additional research-accounter for each (existing GOO), and some more Location/Generic Encounters and Other World Encounters. And maybe add to existing Spells and Conditions, 1 additional card per type, with a particularly more nasty backside effect.

I guess this could come, actually, as some kind of deck box, more than on a small expansion box.
I'd certainly buy it, but I'd expect stuff slightly different than what you said. Obviously, we are different people.
Anyway, this feels like you just want to tell us an idea of how to make the game harder rather than just suggesting a hard-mode expansion.

This game already has a hard mode and each expansion already improves on it. Just only use the Mythos cards with the red tentacles and the game will knock you around.

Anyway, this feels like you just want to tell us an idea of how to make the game harder rather than just suggesting a hard-mode expansion.

Hmm, maybe a bit of both.

Obviously I'd like something nice, shiny and official to add to my decks. I just hope that the game will stay as modular as it is now, with each expansion primarily the Core-Game in mind. I don't mind the odd new mechanic, but overall I'd like to keep my gaming experience as streamlined and un-fiddly as possible. Adding some (optional) encounters which require 2 successes to be rolled, instead of just 1, in order to be counted as a pass, would blow my mind.