The previous version of SystemJS had a separate wrapper project (https://github.com/alexisvincent/systemjs-hot-reloader), that implemented full hot reloading support.

The primitives for hot reloading in SystemJS are the trace API, and the registry delete API, both of which are also available in SystemJS 4.x.

Fleshing out these workflows could be done through the extras model here to define the patterns for a hot reloading API / protocol, or it could be done in a separate project. But it would definitely be worthwhile and useful work.

For example, RollupJS currently has no simple hot-reloading workflows, where SystemJS + preserveModules in Rollup would be a nice standard hot reloading process there.

Please do join this discussion if you are interested in this topic further.