There's no way to predict what things will become collectible in the future. But it's easy to predict what will be worthless, uninteresting, and uncollectable -- anything that is advertised as a "collector's item." To me, people who collect things for the sake of collecting are going about it the wrong way. Collecting is a symptom, a side-effect, a consequence of an obsession. Collections happen because the collector has no choice but to acquire those things that call to him or her for some highly personal reason.
No book captures the spirit of the true collector more accurately than In Flagrante Collecto (Caught in the Act of Collecting), by Marilynn Gelfman Karp. From her introduction:
There are people who amass gemstones and gold. There are competing collectors of baseball cards, stamps, and art. Then there are those who love the unloved. These are the purest kind of collectors; the ones who value misshapen pretzels, Do Not Disturb Signs, and other people's shopping lists. There is deep and satisfying value in collecting arcane objects that most people overlook or dismiss. This book is a tribute to the realm of those "unloveable" objects -- those that seem banal, common, corny, in bad taste, too familiar, or just invisible.
The 1000 photos give testament to the often beautiful byproducts of a having deep and unexplainable curiosity. Link