It would be great if work were started for Java fibers (Project Loom).

There's https://github.com/petrbouda/jlhttp-server-on-fibers/ which seems like a benchmarkable comparison for a minimal HTTP server with and without Fibers. There's also https://github.com/forax/loom-fiber showing deliberate experiments with Fibers and continuations.

It'd be great if there were a branch-by-abstraction technique to composing the core jar from multiple bytecode targets in order to not break backwards compability with pre-loom JVMs. What do I mean by that? Well, XStream goes to a lot of effort to make something that fits all JVMs by using activation profiles - https://github.com/x-stream/xstream/blob/master/xstream/pom.xml