Sumac: I'm seeing it everywhere recently. It's a fairly underrated and unused spice in my opinion. Its tart and subtly piney flavors come from the ground berries from a special kind of plant that produces bright red and purple berries which when ground up produce a flavorful spice. You can use it in place of fresh lemon juice or add it for an extra bit of tang in a dish. Often found in Turkish cooking, but the spice grows all over North America as well.
Buddha's Hand: Now that a few green grocers and supermarkets carry them (though very, very poor examples not worth buying) and a few farmers are planting a tree or two, it's now something that you might be able to find. Its floral and sweet lemon flavors make it great for zest, and since the pith is sweet the fingers can be chopped and eaten on their own which are often dipped in sugar first. Plus the name and appearance are more than enough to sway a few people to give it a go.
Wattleseed: With a flavor that's a bit like coffee, hazelnuts, and chocolate this Australian spice is up on my list. More of a dessert spice, it may take a while before people really start to play with it. Mainly because the stuff is still hard to get in the U.S. unless you buy it online.
Cheese: I know I'm being general but I have a hunch that more people are going to start getting into cheese the way some people do about wine. Profiling regions, countries, type of dairy, and developing pairings and dishes utilizing only the best kinds of cheese possible. It's a dairy revolution.
Figs: When the season comes back people will be all over them. All the recent more popular cookbooks have all had figs featured in them; The Perfect Scoop, Urban Italian, A Platter of Figs, etc... It seems that once the season comes back people will probably scramble to them to start learning how to utilize this honey-sweet fruit.
Any other theories or ideas?