do you ever think bella and edward would join the volturi?
probably not. not in any universe where edward is changed by carlisle and bella by edward, anyway.
I think if canon!edward was ever gonna join the volturi he would have done it during his Murder Rumspringa back in the 20s. that was the perfect chance—he had just rejected carlisle's way of life and "authority." but he still didn't go to italy! I'm guessing he saw aro's leadership as just another smothering trap.
other than that, I can't really think of a circumstance where edward joins the volturi unless they somehow get to him before carlisle does. NM showed us that even if bella were dead, he still wouldn't want to join them. that's a pretty definitive no.
bella...is a more interesting case. I think in an edward-free world, she would be sorely tempted to join the volturi, especially if they offered to change her. she's basically the Good Guy Protagonist™ version of Gianna, when you think about it—a vampire groupie desperate for immortality. (a guillermo, as they say.) she's also desperate for a Family and to Fit In, both things the volturi take pride in offering.
even in a world where edward does exist but breaks up with her and doesn't return in NM, I can see her finding her way to italy someday. hell, she talks about convincing alice to change her so she can spend eternity stalking edward (a line we don't talk about NEARLY enough—fucking yikes, bella.) I can just imagine her only applying to colleges with study abroad programs in that part of italy, feverishly saving and applying and researching local disappearances in the hopes of finding those "civilized" vampires she heard edward talk about. landing herself on their radar by publishing a vampire conspiracy blog that's just a little too accurate. either finding herself in their throne room with a lot of 'splainin to do, or having her neck quietly snapped by felix some dark night when she least expects it.
anyway I'm 100% sure people have written fic about both volturi!edward and volturi!bella if you go looking. if anyone has recs, feel free to chime in with them! y'all know I don't read edbella lol