re: Hubbles next journey?
Well, it was thought when shuttles were flying more regularly, but....
Devoting a mission just for that even then seemed too much!
A shuttle flight is already so expensive.
The main reason why it's not worth it is that there already are devices that had been aboard the HST and that came back during earlier repair missions...The former solar panels, the WFPIC1, etc..
There's at the NASM a scale 1 test model that can house those instruments.