Yeah, Lux Invitica was like jumping in a shark infested pool at the deep end - I have no doubt a lot of work went into it, but it felt more like fantasy than alt-history to me and didn't have the charm of almost-but-not-quite historical events with characters around you taking slightly different decisions. If I had to pick a date for that to happen, I'd say the Christian councils of around the 4th century (having them support Arianism or something), or the death of Julius Nepos, or maybe Charlemagne being assassinated and his line becoming extinct with his lands then being eaten up by Romance-speaking warlords (ie not Franks, but Occitans something). From there you could have reasonably similar gameplay, but vague changes. Islam probably wouldn't exist as a distinct entity if Arianism was all the rage (imo it would have merged into mainstream Christianity, Mohammed would have had nothing to restore because it wouldn't have been corrupted), Spain may well be speaking Gothic, the Zoroastrians and their kin would probably have a lasting influence and may well have subdued the Turks, and quite possibly an obscure culture could have filled the void. If someone dug out historical studies to support EVERYTHING, I'd love the mod. I'd pile code and ideas down upon it like there was no tomorrow. It'd be like the Winter King (starts around the death of Julius Nepos) on steroids.