People are certainly welcome to build an R kernel for the notebook. It could either be written in R, or you could write it in Python, subclassing the existing machinery to evaluate code with R using a library like rpy2.

Personally, I find it more interesting to mix languages in a single notebook - we have a %%R cell magic which allows you to have R cells in amongst your Python code, passing data between the two. An R kernel would only run R code.