A topic is relevant if it’s related to R package development, and if it’s not covered extensively elsewhere: unless a blog post can provide more context, presentation of newer tooling, a new perspective, we are not going to duplicate existing content. We are quite lucky that nothing is too niche for the R-hub blog. 😁 There is also no such thing as a too basic or too advanced topic: for advanced topics, the introduction should make clear it might be of interest to a few readers only; for supposedly basic topics, we hope there will always be something new to some readers.
Then, reading places where people look for help about package development, namely the R-package-devel mailing list, the R Package Development category of community.rstudio.com, the rOpenSci forum and Twitter can help uncover interesting questions.