1)Rebuild the core application as 64bit.
Make the supplied content an option to selectivly install if the users want to (some of it is ok).
2)Instead of the Library use folders & directories just as per the OS you are using, its what people are familiare with after all.
3)NO relative addressing.
4)Hold off on the luxuries like simulated cloth, hair and other affects until the core system is stable. If features are added before the core app is done its going to be the same old mess.

Be up front and admit that this takes time and money, ie set real targets that are achievable and achieve them.