To generalize a little, you are suggesting to backport a Scala 3 feature to 2.13 to give it more exposure. I see that this would be a practical way to test a new feature broadly.
However, this raises the question what features should be backported to 2.13 - enums were already suggested in this thread. Deciding what features are worth testing in this way is really non-obvious. I fear that the selection would be heavily biased by the feasibility, implementation complexity and risk for regressions.
There is also a chicken and egg problem: we want to backport a feature only once we know how it’s going to work in Scala 3. Many features I really wouldn’t want to backport to 2.13 right now, given the pace at which things still evolve in Dotty. But then it’s too late for testing and evaluating: it is now that we need to gather feedback about the new features, to get them finalized and decide if they’re viable.
I also have the feeling that backporting features would give a wrong incentive. Our goal should be to support projects in migrating and cross-compiling with Scala 3, to actually test Scala 3, the interaction of new features, the robustness of the new compiler, the quality of the backwards compatibility, etc.