I love my long denim shirt. You can wear it with jeans, tights, leggings, shorts, skirts, with or without a blazer, over whatever you wear as a long, open jacket (even over dresses), or simply on its own (since it's long).
I also love scarves and jewerly. Scarves can be worn in different ways (ribbon tied scarves and infinity scarves are my favorite) and quite opulent jewerly can be worn even with casual clothes (for example, a plain white T-shirt, boyfriend jeans, pointed stilettos) to "brighten it". Both of them can change the whole outfit.
Depending on the season, and weather. I can only recommend is checking out the lasted upcoming styles that stores have to offer. Specifically the places you would check for places like Rue21, Forever21... ya know?
Handbags, jeans, belts, shoes, jackets are the obvious choices. I have a few t-shirts that go with loads. I tend to have outfits I can dress up or down just by changing the shoes, tights and/or accessories. There are some basic tops I've had for years though that I'd be lost without, like a simple black form fitting top.
Black high waisted skinny jeans. They can be worn with crop tops, tank, blouses, etc. They're casual with ballerina flats or combat boots, and super dressy with heels. They can look classy, grunge, etc - very versatile.
Mine is definitely my off white lacy camisole. It is my absolute favorite as it totally fits my style and is my outfit completer. It can be used causally or formally, ober a tank top or even a dress. definitely a great fallback to have