I read somewhere and long ago that the "Thrawn Trilogy by Timothy Zahn" was ordered by George Lucas to make movies out of it, but that project was dropped later.
Did someone have any reference of that?
I'd heard a similar rumor about the Dark Empire comics published a few years earlier. That led me to the following:
...but Lucas himself actually praised Dark Empire, saying that it was the closest thing to his idea of a sequel trilogy.
So I'd say no, unless Lucas changed his mind quite a bit about what his idea of a sequel trilogy should be between the publication of Dark Empire and the Thrawn Trilogy, I don't think Thrawn was a special order by Lucas.
WARNING: Unless you've already read Dark Empire, that link contains spoilers.
There was a fan base call for Heir to the Empire to be the sequel trilogy, and the HTTE trilogy was, in about 1995, on the canon list. Lucas was, at the time, working on a Star Wars project, as well.
Lucas didn't announce that he was looking for a sequel. Timothy Zahn, in an interview, was asked by the interviewer about the possibility of the sequels being his novels, and acknowledged the possibility, but also noted that Lucas hadn't gone there yet.
It also should be noted that Lucas was prepping preproduction at that point for a new project... we would later find out that that prep was for the revised edition of the original trilogy, and not a sequel. The success of the revision led straight to the prequels.
It should also be noted that Zahn also noted that shooting HTTE would be rather difficult due to the advancing age of the principle actors, and that he didn't think Lucas wold go there.
So, while there was a strong internet rumor, it was based upon wishful thinking, and incomplete knowledge of Lucas' then still secretive project.
I heard a rumor back in the nineties that Zhan's sequel is the official one, the one that is not fan-made but something that is coherent with Lucas's visions and universe. However, I didn't hear any confirmation though.