I'm all for content that draws from the Mario and Luigi RPG series, so as soon as custom moves were revealed I was praying for something like that for both Mario and Luigi's custom moves or maybe even a move where Mario kicks out a green shell rather than shooting a fireball. Also Sonic has sooooo many games he could take from in terms of his moveset, I suppose they wanted more generic and recurring options in the series. As far as custom moves for Sonic, I originally envisioned maybe one or two moves based off the Wisp abilities in Sonic Colors. I'm having trouble remembering some of the better and more original Sonic abilities which could have been moves... oi vey.
Well, he does have the Bounce Attack as a custom Neutral Special, so that's a thing.
Ideas I had for cool Sonic custom moves:
Side Special: Light Speed Attack: A charge-and-store move where Sonic suddenly blasts straight forward in his ball form when the charged attack is released. Has very high KO power at the start of the dash, but a blind spot directly in front of Sonic. Can be jump-cancelled partway through, otherwise Sonic goes into helpless fall. There's a catch, though: This move has no uncharged variant, you have to charge it all in one go, and it takes quite some time to charge. You can cancel out of the charge at any time, but if you do so you have to start over from the beginning!
Neutral Special: Light Speed Attack: An alternative interpretation of the Light Speed Attack, this time as a Neutral Special variant. This move appears as a chargeable Homing Attack. Uncharged, it performs a single Homing Attack that behaves the exact same as normal with the exception of greater startup time than normal, as well as reduced knockback and greatly reduced damage (probably around 4% per hit instead of the standard Homing Attack's 12%) compared to the standard version. But when you charge it, Sonic locks onto the same foe multiple times. This means that when the first Homing Attack hits, Sonic will immediately follow up with another Homing Attack with next to no startup time. At full charge, you can batter your foe with a vicious onslaught of 5 successive Homing Attacks! Each Homing Attack in a single Light Speed Attack activation true combos into the next. This move is ideal for battering offstage foes over the horizontal blast lines, provided the foe gives you time to charge it. It is a little unpredictable as to exactly where the victim will end up, however; just like a standard Homing Attack, the direction of the knockback is the same as the direction Sonic had been moving when he struck the target, and of course the victim is able to KI every single hit.
Side Special: Sonic Boost: A damaging command dash somewhat similar to Palutena's Super Speed. Sonic blasts forward in a run, creating a sonic boom where he took off from just like in Sonic Unleashed and Sonic Generations, but over a smaller area instead of covering the whole screen. This move immediately takes Sonic up to his maximum run speed, and he deals decent damage to any opponent he hits while Boosting. Additionally, foes hit by the strangely-visible sonic boom on takeoff are dealt 1% damage with launching knockback but no KO power. Sonic can jump during a Boost and remain Boosting, but only gets half his normal jump height and deals reduced damage and knockback while in the air. Sonic continues Boosting as long as the Special button is held, and cannot change direction while doing so. The problem is, the move's endlag gets exponentially longer the longer he continues to Boost, leaving him wide-open afterwards if not used carefully.