I'm sure most of this is already known, but here's some on my observation on Link's down air:
Link can autocancel his dair from a fullhop double jump. You input dair right as you use your double jump, causing you to float back to the ground with a dair. If you dont fastfall, it will actually autocancel when you reach the floor giving you zero lag. If you hit someone midair, you can either followup or fastfall to the stage with no lag depending on your height.
Because the dair is out for so long, there are some interesting things that happen when it connects with an opponent. If you hit a shielding opponent on the stage, because the dair is so close to ending, it will actually put you right into your normal fall animation allowing you to do something like nair, or simply back off. (remember that you dont have double jump here because you used it) If it actually hits an opponent on the ground or very close, you will again have all of your aeriel option, meaning you can actually combo the attack directly into nair/bair/uair.
I've done similar dair combos where I'll dair someone and bounce, hit them again and then combo that into nair/bair/uair. This is extra nice because, unlike the example above, you get to use your double jump and it gives you more consistent followups. Keeping track of when your dair is going to end and taking advantage of that for followups is key.
Dair also has ridiculous priority. I did this autocancel dair trick above a Villager and he used upsmash (an incredibly disjointed hitbox with multi-hit properties) and I simply pogo'd right off of the attack's hitbox taking no damage. That's why I feel like this trick could actually see some competitive use, since dair is such a tricky move to beat - having the ability to eliminate it's biggest flaw (landing lag) has a lot of potential despite throwing away the element of surprise when doing so. Doing this in tandem with bomb throwing can create a strong evasive/runaway game that will be hard to approach. Also, autocancel dairs may very well be easier to do when falling from being launched, or on stages with platforms, giving you more surprise and also possibly your double jump.