In an ideal world, the Issues team would develop and support a plugin ecosystem for the tracker. Users can customize and add the features they see fit and even create their own custom plugins. Allows flexibility and power for those who want it while keeping the core simple and clean

This is the best idea.

While some features like +1s and contributing guidelines should be baked in, other features like extra fields, requiring people to sign a disclosure, etc would be best left to plugins that can hook into issues much like CI services hook into pull requests.

This is exactly my fear as well. I really, really like how simple GitHub issues are. While I have some very minor quibbles, I'm also afraid that change here will end up being worse, not better.

I don't thin something that size would be warranted. But, letting users customize Issues, and PR/merge rules, and splitting off another section for requests would be beautiful.