Are there any plans for RIO?
In the long term yes. When we switch to System.Net.Sockets it will be done with an emphasis on transport agnosticism. [1]
ATM, it looks like Sockets will be the priority since it's a cross platform API more similar to what libuv currently provides.
Some recent basic testing show .NET Sockets perform at least as well as libuv. We are strongly considering transitioning Kestrel to Sockets at the same time we migrate from Streams to Pipelines. [1] Pipelines are basically push based Streams that require less total allocations to use. This is because Pipelines allow better memory sharing between producers and consumers.
[1] https://github.com/dotnet/corefxlab/tree/master/src/System.I...
Disclosure: In case it wasn't already obvious, I work for Microsoft on Kestrel.