I'm glad you asked. First, I would make instant a supertype rather than a card type. There wouldn't be instants and sorceries. All nonpermanent spells would be sorceries. Then the subset that now are called instants would be instant sorceries. Permanents wouldn't have to have flash. Oh, that's another keyword I forgot to mention. It allows you to play permanents when you can play instants. They would just have an instant supertype. There would be instant creatures, instant artifacts, and instant enchantments. This would allow us to have one less keyword, make rules text less wordy and finally have a single word for nonpermanent spells."

https://magic.wizards.com/en/articles/archive/making-magic/starting-over-2015-01-26.

I'm not sure where the idea of interrupts with flash came from, but both the example and this article mentions giving sorceries flash.