I know somebody might jump me in the back alleys for this, but why not also combine Castlevania creatures/graphics/music into Ravenloft. They seem like good partners.
Eh, while they share some similar themes, motifs, and tropes, Castlevania is different in tone and pacing from Ravenloft. Castlevania would have been a better fit for the Gothic challenge a few months back. I'd prefer to see stuff specific to the Ravenloft setting in this challenge. However, I'm not in charge of this so do what you like. 
I'm a fan of both and could probably find uses for any Castlevania stuff anyway.
About copyright I think you could use that as long as you don't obtain any economic benefice. But I don't know too much about such things....
That's actually incorrect. It's up to the copyright holder to enforce their copyrights, and they usually don't bother with small potatoes stuff like NWN mods, but it depends on who owns the copyrights. In the case of Strahd's Possession, that game was produced by SSI, Inc., which was eventually purchased by Ubisoft in 2001.
You could sidestep the issue by making an arranged version of the music, similar to what the folks at Overclocked Remix do with music from other games, which falls under Fair Use.
I believe that some of the D&D rules change in Ravenloft--such turn undead becoming more difficult. Have these rules changes been implemented by somebody? (I'm just curious is all.) Thanks. 
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We did scripts for this over at Ravenloft: Prisoners of the Mist, the PW server I DM at. Basically, all Turning attempts are 1 DC higher by default, and no Turning attempt will ever destroy undead.
Turning can be harder in particularly evil places called Sinkholes of Evil. These are places like haunted houses, the site of a grisly murder, or the lair of a darklord like Strahd. This is accomplished by placing an integer variable on the area (SinkHoleLevel) and giving it a number from 1-5. When the script for Turning Undead is run, it checks the area for that variable and increases the DC by that amount.
