I think wonder wing should work something like this:
Pressing down B will active wonder wing. It's a toggle so you can press it once to start, no holding required. Kazooie wraps her wings around banjo. The duo can freely move during wonder wing. They can walk, run, jump, and double jump perfectly fine. They're completely invulnerable to any attack that hits the front. Any projectile that hits, is reflected back. Any attack will cancel wonder wing. To keep players from spamming it, a bar is shown next to their icon. (Similar to robin) This bar will fill up over time, or when the duo takes, or gives out damage. Once the bar depletes, wonder wing deactivates regularly. No shield break animation or anything that punishes running out the meter. However the bar will take slightly longer to start filling up if depleted all the way. If someone is hit by wonder wing, it will do a decent chunk of damage, as well as pop them upwards for a follow up attack. So banjo can dash foward, and follow up with a f-air or n-air.
This is not only a faithful recreation of wonder wing, but gives the duo a good approach option as well.