I'd argue that Bootstrap the framework isn't overused, however default bootstrap theming is.

There's no argument. You are just correct. Saying Bootstrap is overused is like saying Wordpress is overused which does not matter. The fact is, as you said, the default look and feel of websites using these frameworks is overused, not the technology behind the websites. The backend should be overused.

Exactly. Something as simple as using different typography or changing button style defaults would go a long way.

I understand the idea of using something more minimal for just grid etc, but for me, it hasn't been enough of an issue to not use bootstrap.

I don't see this solving a problem that doesn't already have a (few) solutions. It is early and I will add this to my radar