Here is a few suggestions, they kind of go outside of modeling but deal with it as well.
Focus on smooth gameplay, actor movements, possibly a way to get more of the players to be companions, Mainly focus on the single player aspect of the game to bring it back in full such as the missions and things you can do.
Model texturing and smoothing, level editing to create the same enviroment with better graphics, better movement and keep the idea of either first or 3rd person camera view, a way for players or modders to easily replace music with that of their own, for example i changed the asylums song to Forsaken from queen of the damned, and replace some of the not so gothic pieces of music like in vesuvious.
new weapons, new items, new occult relics, possibly the ability to hire a driver and have your own personal car, as well as the taxi as usual, possibly allow players to choose a " pet " such as a gargoyle or ghost to protect and help them.
Just some ideas, I myself have experience with unity3d even though it is limited.
If i may, i'd like to help in the development of this game, as i know the folder structure and original layout of the original game.