20,000 Leagues is being envisoned by Disney as an origin story for Captain Nemo, and that nifty little colon after his name indicates that they'd like to make him over into a nautical franchise to complement their ongoing Pirates of the Caribbean series. Everyone is kind of hoping they'll draw on Jules Verne, and explore Nemo's background as given in that lesser-known sequel, The Mysterious Island. He was revealed to be the Indian Prince Dakkar, who had lost his wife, children, and kingdom after participating in the rebellion of 1857.
While the film is being labeled as an action-adventure, I think it's extremely likely that Wallace is being brought on board precisely to flesh out that tragic backstory, and up the anti-imperialism. This is the screenwriter of Braveheart, Pearl Harbor, and The Man in the Iron Mask, after all. If he has a specialty, it's perilous wartime romances, especially if they're set against the backdrop of revolution. We're probably in for a very frothy 20,000 Leagues, but at least its steampunk setting allows for that. Now cast Naveen Andrews so it'll be sexy steampunk.