Came here to post some news and coincidentally happened upon that post, which I had completely forgotten about. Well, they're apparently back to considering a "bridge movie." Don't offhand recall where I heard about it, but here's the IMDb page. It's said to cover "Aragorn's quest to capture Gollum between The Hobbit and Fellowship of the Ring ... to keep the Ring's location [a secret] from Sauron," Philippa Boyens is listed among the writers, and so far, looks like they also have Andy Serkis onboard (as both Gollum and Director), plus Ian McKellen and Elijah Wood (yes, as Frodo) -- but no mention of Viggo Mortensen.
They also say "The film explores Aragorn's early adventures as a ranger," so maybe they're hoping to avoid the dreaded Elrond-Legolas Syndrome, where a character looks older as you go back in time. Mortensen hasn't been in a Tolkien movie since 2003's Return of the King, and has meanwhile aged from his mid-40s to to his late 60s, a bit old to be playing a "young" Aragorn. I'd guess they're looking to cast a Mortensen-type actor who's somewhere around 30.
As for the three actors listed, Serkis will doubtless be doing Gollum via motion-capture again, McKellen can simply forgo some of the aging makeup he used last time -- and I have absolutely no idea what Frodo would be doing in this story.