Each and every collectible item in the world has its ups & downs. Absolutely nobody can predict which direction the market will effect there specific item in the future.
I think its a pretty safe bet that Honus Wagner good/mint condition baseball cards and any of the SS100 Brough Superior Motorcycles once owned by T E Lawrence (Lawrence of Arabia) are going to do pretty well in the future......but even these treasures are subject to the whims of the market place.