I'm toying with the idea of using FX for a Warhammer (The Old World) campaign. Off the top of my head, I will need necromantic magic, rules for madness, disease, mutations, and halflings.
Are any of these covered in FX or the Express Additions?
Halflings are listed in Express Additions #2.
Necromatic Magic is part of the Diabloism School of Magic. (at least basic Necromatic magic is) starting on page 150
Poisons and Diseases are in the core rules, starting on page 218 (it just covers the basics and gives a few examples)
Madness - while there are no specific rules yet, several of the Background Drawbacks could apply starting on page 73, such as Phobia, Paranoid, Psychotic Temper, plus the Winds of Insanity spell (p. 153)
No specific rules for Mutations, but it might be possible to adapt the Warhammer rules for such (not positive on this, since I have never looked at Warhammer specifically (there was once a board game that I think was based on Warhammer, I have that game but have not looked at it for years....)