Adding an air shield could do really good things for Brawl's defensive game, but you'd have to make sure you balance it on the offensive end as well.
For example, if you just take SSBM and randomly toss in an air shield, you basically give every character an all-purpose flying god-block, which would be incredibly lame for all sorts of reasons. With the addition of the air shield, there would also have to be some way to penetrate the air shield, such as an air grab. Whether or not we'd want air grabs in SSBB is another discussion altogether.
Also, even if the air shield could be countered by some kind of air grab or other unblockable attack, it would still be an all-purpose mobile shield. That's huge, because unless your name is Luigi or the Iceez or some such thing, you can't move while you're in a ground shield. A mobile aerial shield would definitely be capable of ruining all sorts of juggles and giving new and potentially broken meanings to "Wall of Pain," which wouldn't necessarily be fun either. So the mobility of the air shield needs to come at some sort of price. For instance, an air shield should shrink faster, suffer larger shield stun, have greater shield knockback, or have less counterattacking options than the ground shield.
If I were designing Brawl, Brawl would be a very strange game indeed... but anyway. In my twisted vision of Brawl that would include the air shield, the air shield would figuratively transform a character into a giant beachball. Basically, you would take no damage from attacks, but you would still suffer significant (if reduced) knockback, meaning that the opponent could kick your air-bubbly shield *** about the stage--or even off of it--if you don't block intelligently. This creates some interesting juggling and edgeguarding scenarios, because suddenly you have the option to take no damage, but you have to decide whether it's worth giving the opponent so much control over your location onstage. Add in the risk of getting air grabbed and thrown off would only add to the fun.
... My head hurts a little, but now that I think about it, I think an air shield could work really well.