Hello and welcome!
I'm not sure off-hand about set threads talking about R/C writing samples (hopefully others can dig some up), but I can speak briefly here to my own experiences. Even though I had plenty of R/C writing from my BA and MA in the field, I decided to write a brand new writing sample in December, from a seminar paper I was writing. So, not a lot of time, and even though everything worked out for me, I wish I'd have had more time to think through my ideas. It's smart you are thinking about this early.
I asked my professor, and she supported the idea because the paper represented something I was really interested in and that (admittedly) was pushing a few theories and movements. Conventional wisdom says you should try to pair your WS with your personal statement/established research agenda and identity, but I don't know if that's necessarily what you HAVE to do. Some programs really just want to see you carry an argument and demonstrate knowledge in your field, even if that argument is not linked to what you want to study. No one will EVER (or should ever) hold you to what you write about in your WS and personal statement anyway, so I'm generally a fan of submitting what you think is your best work/most interesting work/etc.
For my MA, I submitted something that demonstrated my knowledge of rhetoric; for the PhD, it was all comp/WPA. This definitely might depend on which program you are applying to (some places may need more of a specific sub-field than others, or you may have more competition in one sub-field than others), but I feel like your WS helps the adcomm "brand" you in that way, too. For one school, I was literally put in the "WPA pile" bc of what I had written about in my WS. Part of me wonders if I still would have gotten in if I had, say, gone all in with rhetorical history or something. You just never know.
Just a few thoughts I have about it! Again, hopefully someone can dig up the wealth of threads that are likely already in existence.
