ADG,
Not familiar with most of the resources you mentioned - but Thomas Harlan's series immediately jumped to mind after reading your post.
Depending on "when" you plan on setting the campaign, a couple of thoughts include:
1) A Great Evil Arises in the East - Persia/Parthia, rocked onto its heels by rapid Roman expansion, turns to the mysticism of their ancient forefathers. This results in the awakening of a sleeping evil (imprisoned since the days of Gilgamesh) and the clash of the Cthulu-driven Parthians with the expansionistic Romans (Late Republic/Early Empire)
2) One of the leading families of Rome, defeated and dispossed during the Roman Civil wars, enters into an unholy pact with the ancient evil. With new powers, they rise to power, crushing their opponenets and "seizing the Purple". They transform the budding empire from one driven by Roman ideals to an oppressive monster seeking the ultimate subjugation of all corners of the Known World (kind of alternate history, perhaps using Julius Caeser as the main antagonist - Late Republic/Early Empire).
3) Pax Romana has reigned for 100 years - yet darkness nibbles at the edges of Empire. A mysterious cult - whose members display impressive powers - has risen in the East and now boasts cells at every Imperial border. Nameless horrors cross Hadrian's Wall in Albion, dark fears grow in Dacia, the Levant is plagued by demon-spawned desert raiders and an unknown power is poisoning the headwaters of the Nile, threatening the Empire's grain supplies. Evil is on the march (Mid-Late Empire).
~ Old One