Yeap, trends baby trends...
I hear a lot of antique dealers, auction houses, and estate sale companies lamenting about how they can't sell furniture anymore. I can tell you why, (Atleast in my area) A LOT of people are passing away now who owned bland, dark tone, American furniture made in the 1950s & 1960s, so large supply on the market. In my opinion, there are very few customers (low demand) for bland, dark tone, American furntiture made in the 1950s & 1960s. Simple as that.
As far as guessing the next hot collectibles, I would guess toys, video games, music, from the 1990 to 2000. I'm gonna guess to especially be on the look out for things that actually weren't very people then, but most people recognize. Think Milli-Vanilli & Vanilla Ice merchandise. You could probably pick up stuff like that at flea markets and yard sales for pennies.
Remember, only certain things are gonna get hot again, not everything, and nobody knows whats gonna hit, but if you have the storage space it might be a good investment to just start collecting from this time period and stacking it away. New or like new condition is always going to be your best bet.