Well, the attack where he flies up and stomps the ground will be the Down B. The attack where he scratches the ground with his tail will be either Dash or Side B. The famous "coming in from the background" will be the FS. N-B will be a fire ball. Okay, there are some moves I like in the Crusade moveset, but the intention is not to copy it. ^^;
Well in most games featuring Ridley, his attacks range from projectiles to a lot of Tail swipes.
In the earlier games his Tail was the only part of him that really attacked you (Except Mecha Ridley in Zero Mission's end game... oh yeah spoilers...). In keeping with that feel, I suggest that his aerial moves use the tails.
If you gave him low gravity he could stay in the air and use the attacks the old bosses had.
I suggest the following for this idea:
Aerial attacks:
Forward A: Ridley whips the tail from behiind him, to underneath him, to in front of him. The area of hit would be underneath and in front of him. Hit boxes underneath would send players downward. Hit boxes in front would send players just above him to allow him to attempt and aerial combo with the other aerial moves (Say you hit them up just in the line of Ridley's mouth to be hit with a fire ball, or if they go too high you can use one of your jumps to use another aerial move(s)). Due to the ease of use with the attack, it would have so-so damage (maybe a sweetspot of damage at the very tip of his tail) but wouldn't have too much knock back. This attack is a "set up" move while also acting as protection from underneath him. Players need to work around this move, as you would with the actual boss. This is also done because if you give him low gravity, pressing down should allow you to descend. So rather than have a swipe for down A and Forward A, I basically combined the expected attacks into one.
Down A: Ridley has slammed downward before. As basic as it is to have an attack like this, it suits him, and it is actually used in the Metroid games. So pressing down in the air will cause the low gravity to become higher and that'll make him descend. Pressing down A will cause a sudden slam to the ground.
Up A: Upwards chomp. Ridley has attempted to bite Samus before, and it's no surprise considering how big his mouth is. It extends outward farther than his arms can in most incarnations, so it's only natural to have some mouth moves. He is animalistic afterall. He's going to use what his body allows him to, and with a head of that size, I'd say it makes sense. If you don't like that though, then perhaps an upward swipe with his arm, like Wolf? Or an aerial summer-salt with the tail being swung around like a flail during the maneuver?
Neutral A: I believe this one NEEDS to be a swipe with the arm. Ridley pivots his body and swings it forward, like Wolf's sort of, except slower and with a wider attack range. Ridley has used his arms IN THE AIR in the 3D games to grab Samus and swat at her, so it goes with what is known to occur. You could even make it kind of like Mario's where it sends players downwards. Then follow up with a down A to smash them into the ground.
Back A: Have Ridley flap his wings once forward, forcing him backwards. Think of a back slam. It wouldn't have the KO power of most back kicks, but the hitbox would be long, as his body is big. The sweetspot should be on the pointeist part of his back, as his size makes it difficult to coordinate that sweetspot. This one spot should have the normal back kick KO knockback.
If you decide not to use the low gravity, most of the moves still work well in theory.
As for the ground moves... will he be floating as he is during the fight, or are you going to make him stand? If so is he bipedal or quadrupedal? In almost all incarnations, he's quadrupedal.
