I would pick up a pack of basic white/black papers, maybe a solid color pack? I know there are a few good black/white from different designers: Forever Joy, Pink Reptile Designs, and Little Butterfly Wings have one for sure. Amy Wolff has great colorful solid packs as does Allison Pennington. Oh, and a pack of tiny patterns, or a kit that has tiny patterns. Because I like them and find them versatile.
I would pick up a few alphas. Maybe a good black/white and one that has lots of colors in a style I like.
The Collab idea is fabulous! There's so many awesome pieces in the store collabs and they are so full!
I would grab a set of stitches (if I didn't already own so many packs!

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Other than that, I would say look at photos you have and see if there's particular themes or colors that you want to scrap. Those don't always fit the MOC challenges, but it can give you a place to start!