The goal is for the next version of Up to provide all of the runtimes which Lambda already supports. Up lets users write regular HTTP servers, using any framework they prefer. Up works by translating Lambda events to and from HTTP requests, using a Go proxy inside of the Lambda.
Currently Lambda imposes limitations which prevent this from working well with all runtimes, since the Go proxy needs to be the handler, while the runtime files/interpreters still need to be present.
I see two solutions to this problem, the first is to expose the existing runtimes as layers, allowing the proxy to live in a "provided" runtime, but still reference the others for their files. The second is to allow layers to act as the entrypoint/bootstrap, while still allowing the user to select an existing runtime as they normally would, just giving precedence to the layer.