Although I am not planning on using it soon, I have been toying with adapting the existing core classes to other genre.
The fighter and Rogue of course are truly universal and can be played with any genre by simply changing the proficiencies and dropping the few magic related talents of the rogue (like minor magic) and magic related feats of the fighter (such as disruptive).
In fact, with body armor making a come back in the modern era, and probably only improving for the future, I don't even feel the need to come up with an unarmored fighter variant.
I would also toss Barbarian in the universal vat as well. It would simply represent someone with a tough attitude in eras where real barbarians no longer exist. I would drop the trap sense and toss in a couple bonus feats however to make it fit better into more games.
The Monk is of course the quinessential martial artist who fits in many genres as well. I would cut down on the speed bonus though as it borders on the ridiculous in a modern setting.
With tweaking the Bard, Ranger, and even the Paladin can be used.
For the Bard he becomes a combat leader, keeping his bardic music only now it is stirring oratory. He would lose his spells, but get a few bonus feats (all dex or skill based) and reduce the number of skill points but get a BAB of 1-20 instead of 1-15. He would lead from the front, not singing from the rear.
The Ranger makes a nice scout, bounty hunter, and Gunslinger. Drop the Bond, and the spells, replacing them with 4 or 5 talents stolen from the Rogue.
The Paladin of course is the champion of a cause. Out goes alignment based smite...not just Smite not Smite evil...and out go the spells. The Lay on hands/mercies will only be on himself and represents great recuperative abilities rather than magic healing. To replace the spells I would give a few of our old friend the Bonus Feat.
What does everyone think of these?
I'll go into possible non-fantasy uses of the big three spellcaster another time.
When I did a World War II/Ravenloft game, I used D&D classes for some of the modern characters.
I made the British Officer a Rogue, to indicate his extensive training and his accurate skill with firearms.
The Australian Soldier was a Fighter.
The Gypsy was a Thief-Acrobat. With a +19 Reflex Save bonus she was _almost_ immune to machine gun fire. Unfortunately, she was NOT immune to Barbarians great swords.
I made the Orthodox Priest a Cleric, on the grounds that God specially gave him clerical spells for this journey into Ravenloft.
The British Radio Expert was an Expert.
I made the Hungarian Count a Maverick, which is a class from Deadlands D20. If I didn't have that class, he could have been a Rogue.
Cool, would have made a good Call of Cthulhu group as well.