This is great speculation! However, I think Mistborn is a safe bet for what they'd start with. It's easily his most mainstream and popular work, and probably has the most name recognition. It's what studios want in a opening film: built-in fanbase, name recognition, not overly complicated, standalone with immediate sequel potential, etc. Unfortunately, I don't see Elantris or Warbreaker getting picked up until: a) the Cosmere gets traction as a major cinematic universe, with a one or two successful releases, and b) Brandon releases his sequels.
I think with The Stormlight Archive, what I think would work best as an adaptation is each book as a mini-series of three 90-minute episodes. Despite a shorter running time, I think that would work better for the structure of the story then ten 45-minute episodes à la other big-budget television.
However, if you strip down each Stormlight book into major events, story beats, and character arcs, each one is relatively compact - dense and detailed, but straightforward. I think they could work as films without being completely squandered. Kaladin and Dalinar's stories in book 1 are great and cinematic, and complement each other. Although, I think Shallan's book 1 story is so brief and disconnected, it could be moved to the beginning of a more Shallan-centric Words of Radiance, and have Kaladin/Dalinar appear a bit later into the film.