That's already the case. You can legally import a modern kei car if you can modify it to meet relevant safety and emissions standards. You can import any modern car. I imported a 2001 BMW from Canada a few years back and it was a breeze because it already had the EPA stickers and BMW NA signed off on the safety stuff. You can import a non-compliant car (e.g. some carburetted kei car with minimal safety equipment) if you bring it up to the US standards that were in effect at the time. Nobody wants to bother does not mean that nobody can.

The 25 (well I think DOT is 20 years and EPA is 25 or vice versa) means you can import shit without it having to meet relevant federal standards. States (e.g. California) will still want to see proof it doesn't pollute too much, and CARB tests are expensive. Sates like Washington will let you register pretty much anything with wheels which is how you see a lot of non-US. market cars for sale out here with Washington plates.