Good point. The spec says that blockquotes may be “a single character > not followed by a space” so this could indeed be a problem.
My suggestion would be to make the space a requirement if you’re starting the quote text with a !, otherwise it will be interpreted as a spoiler.
I agree that spoilers should be an extension. Note @codinghorror’s comment in this post about fenced code blocks intentionally being the only significant addition to the core. It seems unlikely that any of the major additions we’ve been discussing will be accepted as part of the core and that’s fine.