I like the grab idea a lot. Perhaps Side-grab could do it. then side b would be batarang. I like the Down B because Batman needs a stealth move and one that can be combo into and out of, also it can be badass if used right. I need a batman weapon/gadget for the B move that Batman uses in most of his carnations and not just Arkham City. I was also thinking that his side taunts could be his cape flurry that stuns for less than a second just for badass looking moves.
I was thinking, Batman's B could be a charged horizontal batclaw. I find the power to grab across a stage powerful and by putting a charge on it, might just be the handicap it needs. I was also thinking there would be an automatic powerful attack right after he pulls you in. This could allow him to do it in the air also.
His glide attack could be a double foot kick and with his down air he should come strait down.
Lastly should I make it explosive batarangs or non or make it like Samus' side B with the fast missile being regular side B and the heat seeking being the tilt side B
And just a cool combo is Up B, Y, Down B, then Down Air
Assuming the enemy is in the air, pull him down with Down B press Y to get launched up then press down b to send a pellet down and stun him and drop strait down on him.