I have read through this thread and I think all the ideas are interesting.
Right now, I'm going through my dark ages, but I have maintained an interest in the online community focused around historical LEGOs (including Pirates).
Here's my take:
The Viking stuff failed because the sets were mediocre. The "main fort" thing was cool, but not appealing, it could be easily replicated, and there wasn't much color (lots of brown bricks)
Secondly, adults didn't like it because Viking helmets did not have horns--and the execution of said horns was not appealing to kids either--or kids interested in Vikings who'd know that Vikings didn't have horned helmets.
That said, I believe there was a 2nd variation which was the two eye protected pieces attached to a conical helm--that was cool.
Additionally, the mythological creatures seemed to be the main attraction of having such sets, and if you're like me, you'd like the minifigs, not the fabricated dragon.
I'm a huge fan of Vikings as I am of history, but the LEGO Vikings were sloppily executed.
Unfortunately, that seems to be the case of a lot of LEGO Castle sets being put out today. There is no real substance to the sets--which is fine for me because I just order specific minifig parts and lots and lots of bricks off the web.
The whole troll and dwarf theme I find to be pretty lame. The only cool thing is the winged helmets which I like quite a lot.
As far as Greece vs. Rome, well seeing what LEGO does now with the Viking, Castle, and Pirate themes, they're going to make something none of us might like. They might not, but that's my gut feeling.
I know if they do a Rome theme, the legionaries will have that ridiculous "lorica segmentata" that Roman troops are stereotyped as having--in reality more Roman troops wore chain mail than they did the lorica segmentata
I'm afraid if they do a Greek theme, they might have the ridiculous 300 theme with no armor for the Spartans and some tight loin cloth or something. That said, a Greek theme would be more interesting imo, if they did it "right"
A Greek theme has the most color, the most potential for branching into fantasy, and a more varied amount of soldiers (knowing LEGO they're probably going to just have the stereotypical "Legionary" Outfit).
I remember playing the Age of Empires 1 demo back in 1997 and then constructing a Watch Tower out of Wild West lego bricks.
Again, the real question is that if they were to do such a theme, would they do it right, and would they do it justice? Obviously, the fans will generally have the best interpretations of the LEGO theme, but new LEGO parts can't hurt.
And gladiators don't really sell me too much--the real gladiators were nothing like they were in Ridley Scott's film.