I honestly don't think 'dumbing down' a game is what casuals are looking for. I think we need to move away from an idea of 'How do we get them to like what we like?' or 'If you're not playing to get better/win, why are you here?' and more acceptance that people play games for different reasons.
Developers should instead think about how potential players can enjoy the game in as many different ways as possible. You don't even need to throw a ton of AAA money at it. Something like a color-edit mode (CvS2 style, not SFxT insanity) or a Kombat Tower, or a World Tour. Something simple, yet addictive. If you have an opportunity to 'share', even better. Look at SoulCalibur 5. Even though the creation mode was hurt by the budget (it walks a fine line between brilliant and garbage), there are still people tinkering with it today. You can still find people making 'show me your creations' rooms online. Are these people not keeping the game in the ether as the people that still travel to play in tournaments? Or stream the game? Obviously, create-a-character is no joke in terms of development, but it shows how including something like that goes a long way.
Juicebox has said that learning a fighting game is like learning an instrument. Some folks would rather play Guitar Hero. That isn't to say they want to take their piece of plastic and hop on Juicebox's stage and jam with him, but they have their own way of enjoying music. Just like people who just like to listen to music and support their scene by going to concerts (viewing streams or going to evo).
If you want people to get used to fighting game inputs, I think Fight Lab in TTT2 was the beginning of something. Turning the act of learning fighting game stuff in an of itself into a series of mini-games with fun visuals and instant gratification. Something like have a Rhythm Heaven medley mode styled mini-game for each character, where their theme is playing and you do their special move inputs with the music and it gets faster and more challenging. It would make doing the inputs second nature, while building an attachment to the game and their character. Then they could move on to other things.
https://www.youtube.com/watch?v=rKvgZI1CJq0
Honestly, I'm surprised a dev hasn't tried making a free to play game where arcade, practice, local vs, and online are free with all the characters playable. Then charge for cosmetics, and story mode chapters that have 1-2 hours each focusing on a character's experience at the tournament, maybe with a Tekken 3 styled Tekken Force stages in between the major fights. Give a chance for every character to shine instead of disappointing bit parts and allow new folks to get attached to the world.