I was messing with this stuff before my break, but I'm basically working on following opponents in the air with nair juggles, best fast fall air dodge to jab times, and using up air for kills.
According to the updated stats topic from tactical discussion boards, Ike is tied for 7th for fast falling. That means that Ike can fall faster than at least 20 other characters, and fall not much slower than the 6 or so above him. That is why Ike should be able to follow people in the air without much effort.
Also, Ike should be able to, at will, fast fall + air dodge at an amazing speed into jab or grab in many instances. I haven't really tested this out much since I returned.
Ike's nair outranges many moves, so against such range, many opponents must try to avoid it with an air dodge, but smart following in the air+good use of nair will help you get successive hits where the opponent can't get away from the attacks. Up air can juggle a little bit, and can get kills.
Nair- First and foremost, I recommend attack stick for ease of spacing with nair.
When you're facing them, it's best to start nair when the oponent is in front and above you, and end it when the opponent is either behind you or "on" you.
Facing away is a little harder(but better for mixups w/ up air). Essentially, you have to start it in the position above (and a little bit behind) you where the beginning hurtbox of nair begins. Then, just keep the opponent ontop of you or very close behind you.
Regular nair followups also apply, such as bair, (pivot)grab, jab, etc.
It's not that hard to hit people that falls faster than you with this (unlike up air)
Up air-
IMO, it's best to use this when facing AWAY from the opponent, because the ending hitbox seems to last longer around that area, and the hitbox seems longer/more portruding (if anyone could provide frame data or w/e, that'd be cool).
It's a bit harder to hit than nair,because you have to keep the opponent in a more constrained position, but you can get really easy kills in the air, and you don't have to worry about many things outranging this. it's very good when people are falling towards platforms, so one can actually get consecutive up airs on multiplatform stages like battlefield.
Teh Brettster:
I would love dtilt a lot more if people were in the right position to set one up, but they never are, so I have to do all the setting up myself

. You could try messing with dtilt I guess, but I think there are just more efficient uses of your time that could be used.