Does it make sense using Next.js/React for building ChatGPT plugins? Or just node.js and express since we just have to expose api routes.
You could do both depending on your app. Pretty sure nextjs is just react + some layers ontop of express. If you need an auth flow you can build the login UI in react and then handle all the endpoints with the backend side.
Otherwise you could just go minimilist and serve an HTML file with jquery or vanillajs if you really are just building only a login page.
So short answer is, you can do either. NextJS would be good if you anticipate the need for a more complicated UI to be built to compliment the plugin. Or you could keep it really simple and use express and serving the one html file for oauth and exposing endpoints.
Hope that helps.