I cannot really fathom Robert Plant. Since the death of John Bonham in 1980, it has been Robert Plant who has been the one reluctant to carry on with the Zep. For the first eight years or so after Zep's demise he flat out refused to acknowledge his work there. It wasn't until his solo album Manic Nirvana that we even got a glimpse of that spark that was the frontman for Led Zeppelin.
Zep fans got some satisfaction in the mid-90s when Plant reunited with Page for their No Quarter album/MTV special/tour. Although, I was disappointed to hear they did not even approach John Paul Jones to join them for the project. He only heard about it when everyone else did and then expressed a sentiment that he would have liked to be involved with it. And the second Page/Plant album, Walking Into Clarksdale, was frankly a disappointment because they STILL did not include Jones after that expressed interest.
So perhaps going on without him is the best way to go for Page, Jones and Bonham TNG. They could put together an all instrumental series of tracks in the studio while Plant is currently on tour. And once he returns home, his ol' pal Jimmy calls him up and asks if he wants to hear what they've been working on. Then, hopefully, Plant gets inspired upon hearing the new material and lays down vocals on the tracks. Short of chaining Plant to a chair in Page's studio and taking a pair of pliers to his genitals to get him to do those high-pitched signature wails, I think that may be what it takes to get Led Zeppelin back together for more than a single appearance for charity.