LOS ANGELES, CALIFORNIA.- Andrea L. Rich, director of the Los Angeles County Museum of Art agrees that the decision to rebuild the biggest encyclopedic art museum west of the Mississippi from scratch was as much a surprise for her as it was for the rest of Los Angeles. "It wasn't in the cards," says the museum's president. But on December 5, LACMA unveiled a stunning plan: Instead of merely remodeling its campus on Wilshire Boulevard, the museum would raze four of its six buildings, replacing them with a vast, futuristic structure designed by Dutch architect Rem Koolhaas and topped by a translucent, tent-like roof. Instead of compartmentalizing its collection in separate buildings spread across the campus, it would consolidate almost everything under that dramatic roof.