For example, I personally much prefer Cartodb’s positron tiles than stamen toner’s (they’re a lot lower visual noise, so trains the eye more on the data than the map). I could make a PR to include that. However, as I was making the PR, I noticed your contrib guidelines asked me to create a bug first. And well, it’s not really a bug per se. Also I’m not sure whether you want more preconfigured tile providers or if bokeh.tile_providers is more intended as a guideline as to how to add your own.
Sorry this message is rambling slightly (late on friday after a long week), in short: would you like a PR to add cartodb positron tiles or not?
see: https://carto.com/location-data-services/basemaps/ for what they look like, if curious.