Esoterrorist Second Edition did more or less nothing to change the system (I think they changed initiative) but since Esoterrorist had one of the tightest systems I think it worked out well. I've run games Esoterrorist, Mutant City Blues, and Night's Black Agents. I've used Fear Itself rules in part and I have read Ashen Stars.
For Trail I don't think there's much to add. If you want more action then just grab rules out of NBA. As for the core mechanics? Unless you want to borrow specific Investigative Skills (Vampirology?) from other games I think you are pretty much set using Trail as written.
If you want to have a more direct spell type or give monsters weird powers you could also borrow from Mutant City Blues. I ran a game based on the vidego game Prototype not long ago using this approach.