Being a mom myself, I think that is a GREAT idea! Don't tell her. Let it be a suprise to her.
Is there a song while growing up that meant a lot to the both of you? Maybe one you got crazy with? That's always a nice feeling..... to re-live a moment you shared with only her.
It's just you and your mom's song.......forever.
Or............create a new one now! Before your wedding date and then use it at your wedding.
I also agree with 2 others replys, Wind Beneath My wings & the Faith Hill song.
You'll be in my heart, Phil Collins (that's me and my son's song)..........GREAT ONE. Listen to the words of that song.
Whatever you decide to dance to, your mom will be thrilled!