I like this idea and could see it being visually represented as a faint red/green bar behind the URL text in the address bar, with a greater amount of the bar being red when the domain is less trusted.

As for developers trusting a plugin that reaches out to an external location to determine the reputation of every website they visit seems like a harder sell though.

that's a good one not perfect for sure, hackers would just start buying domains earlier but still...

Yeah, but there is a takedown process when a spam site is detected (the server provider can shut off access, etc), so it is a game that is somewhat winnable.

There are curated lists over newly registered domain names that some security software uses so it should be easy to add without any privacy issues.