Q: Is there any plan to offer a modern leather band for the 42 mm watch?

I'm afraid that no-one here can tell you anything about Apple's unannounced product plans (this is a user-based technical support community) and it is also not allowed here to speculate.

If you wish, you can suggest new bands to Apple here (all submissions are read, but you should not expect to receive a reply):

https://www.apple.com/feedback/watch.html