I wouldn't know about Harry Potter. I've never read the books or watched the films. I do know, however, that the HP films started production before all the books were done. People who got hooked on the first few books were happy to see film versions and how they compared to the books, and this, in my opinion, was the driving force behind the franchise.
With the Thrawn trilogy, they've been out for over 20 years already. Yes it'd be nice seeing how they'd translate to the big screen but at the end of the day we know how it ends.
It's not like Lord of the Rings, where they tried to ensure the story remained true to the spirit of the books, but at the same time broaden it's appeal to reach an audience beyond the hard core readers. We live in a world saturated with Star Wars; I'd be very surprised to meet somebody under the age of thirty who hasn't at least heard about Grand Admiral Thrawn or Mara Jade.
On a side note, I'd like to see the Han Solo Adventures (Han Solo at Stars' End, Han Solo's Revenge, Han Solo and the Lost Legacy) turned into films, possibly adjusting them to fit better into the existing universe. These are likely books that few people outside the purists will have read (I've only managed to read two of them). It would also give an opportunity to introduce a new actor to play Han, which could then offer a stepping stone link into new dedicated SW movies.
I think you're over-estimating the reach of Zahn's books, and probably the EU books in general. To be honest, I'd be surprised if 10% of people under 30 had heard of Thrawn or Jade.
The books are about 20 years old, and they don't have main stream impact like LotR or Harry Potter.
Yes, the world is saturated in Star Wars. But these days the period after the end of the original trilogy doesn't get that much attention, (especially outside of books).
A lot of the kids stuff these days seems to concentrate on the Clone Wars stuff.
You've got the settings of during the prequel trilogy, the original trilogy, and the period in between.
Then there's stuff quite far back in history (the Old Republic stuff etc.)
Only then do you get to the post-Return of the Jedi stuff.
In general terms, films these days seem to favour going back and doing reboot/origin stories.
The problem with Star Wars is the prequel trilogy already exists.
But as you mention, I think there is scope to something with someone like Han Solo, who wasn't involved in the prequels at all.
(Although apparently an early Lucas draft had a 10 year-old solo on Kashyyyk in Revenge of the Sith)
