Without seeing the dancers you will be working with it is impossible for me to give specific advice. However, here are a few tips that you can answer that will get you started.
- Dance Style: Watch the dancers perform other dances besides the one you are helping them with. Get to know their style and ability. Watching them should generate ideas of what magic you could incorporate into their style of dance.
- Time, talent and resources: How much time do you and the dancers have to practice? This will play a huge role in what magic you use. I will use the first two videos you posted as examples.
The dance with the floating ball had to be put together in a very short amount of time. Therefore an effect was used that did not require the dancers to actually learn an effect. (They danced around it, literally) These dancers also did not have a background in magic so a self-working effect was chosen for them. In other words they were not stealing doves, rolling coins, or doing card fans. This show has a good sized budget and versatile performance space allowing for a stage illusion which required more than a platform in a gym.
The video with the shadow puppetry used dancers hired and trained specifically for that type, and probably for that specific dance. Therefore they created the illusions with them rather than around them. It is safe to say that thousands of hours went into that one production, and it showed. For this performance the illusions were the dance.
Evaluate your time, talent and resources to judge what effects will work best, keeping in mind the style of dancers. You also mentioned their dance has a story. Discover what it is. Get specifics. Often times dancers use abstract images when describing their dance. If this is that case get them to verbalize the feel. Have them explain it in sounds, color and even smell. Another way to get to the heart of the dance is to discover where their inspiration for the dance came from. A painting, the music, a book, a movie etc. Study whatever their inspiration was and use it as a jumping point. Lighting designers sometimes use these methods for creating lighting for the show.
One last piece of advice. For dance performances incorporating bits of magic into them less is more. In other words, if in doubt leave it out. The dance should be the focal point with the magic effects highlighting their story. If it doesnt need it dont use it.
I hope this helps even a little bit. Good luck and have fun.