It all depends on what kind of beers you like to brew.
If say you allways brew dark beers.....Get a bag of chocolate malt, or if every recipe you have has c40 in it...get a bag or so of it.
Personaly i store my grain in a 8 gal bucket. It holds a entire sack of grain and i just put a label on the bucket so i know what i have at a glance.
Also, You could start to stock up on hops. www.hopsdirect.com
Go there and basicly have fun. I went with leaf hops this year for kick and giggles....took a little bit to get used to but they work like a champ.
So basicly, If you can store extra grain, hops, yeast, you have the basic's covered.
Next step after that is collecting more carboys
Or if you are thinking about kegging....allways more kegs.
Hope this helps