That's a best case scenario, sure, but honestly while it's great on paper, it's actually not applicable to real life. As MST3K always said, 'There is no accounting for taste.' Choosing a secondary needs to develop from a space where you enjoy playing that character, and then expanding that to working on what makes that character good in a competitive sense. Sonic is fast, and covers people who don't understand the character very well, but he's got huge deficits in his game. To be honest, choosing a secondary that covers all your bad matchups may simply not exist, or they have a playstyle that doesn't gel with you. It's like if I started playing DK to cover my issues with Fox - he's got a chaingrab, some fantastic range, and fast, reliable aerials to give me an edge in the air. However, I don't like playing DK, and he has very few skills that translate from Sonic.
Sonic is a weird character. The skillset it takes to use Sonic does not give you many transferable options when playing other characters, like Falco to Fox, or Sheik to Squirtle(seriously, give those two a look, Squirtle is aggro Sheik). I've been all around this character select numerous times and found that, in the end, Sonic is just crazy unique.
That isn't to say there aren't parallels. If you want to improve your knowledge of the game in general while finding different ways to better your Sonic along with training some secret weapons, I have a couple suggestions for y'all.
Jigglypuff
First and foremost, learn yourself some Jigglypuff. She has a very similar juggle game to Sonic, and if you treat rest like a sweetspot Nair, you might find yourself going for insane combos and nailing that elusive move a lot more than before. But mostly, you're Bair training here. Sonic might not be able to Bair insanely like Jiggs, but they have the exact same use for it - learn it, space it, kill with it. If you're looking for the horizontal velocity that Jiggs can get with Sonic, try a Moonwalk shorthop bair. Having a Jiggles on hand is a great momentum shift as well, able to freely deal with your opponent's mindset in mid-set. Jiggles used to have a lot of trouble, but in this update she might shine quite a bit more than before.
Game and Watch
Like Sonic's UpB? Well you shouldn't, but here's a much better version of the move that can reliably sweetspot, has a hitbox, and you can act out of it! His ground game has range comparable to Sonic, so you need not mess with your spacing knowledge too much when transitioning from one to the other. In fact, when it comes to range, everything is comparable to Sonic, even if he is much slower on the ground with much different applications to the moves. You can also feel much more free to take an opponent to stages with closer quarters but larger boundaries, as you'll never be wanting for kill power.
Sheik
There's nothing really here to get you better with Sonic, but having a Sheik teaches you patience, which helps you get better in general. Sheik is the character that has low impact tech skill, and a very basic combo game. Patience, spacing, and timing your moves are the most important aspects to a Sheik, like every move is timed almost directly to Sonic's Nair. She also has an astronomically better tech roll situation, so you can at least rely on escaping your opponent's grasp sometimes, as opposed to your sad little hedgehog's pitifully bad tech options. In Melee, it's said that if you want to learn the game's fundamentals best, play Sheik, and that hasn't changed.
Something to keep in mind.
