I have just finished adding support for converting Imperator:Rome migratory tribes as Crusader Kings II steppe nomads and support for the Invictus 1.8 'Midas' update (including its new secret revivalist religion). There are 3 options for steppe nomad conversion: To convert all I:R migratory tribes as CK2 steppe nomads, to only convert migratory tribes with a historical horse-based culture (such as the Scythians) as steppe nomads, or to convert all migratory tribes as regular CK2 tribes (as was the case before the update).
Steppe nomads in CK2 have a few interesting quirks compared to other government types, which necessitated a different approach compared to regular countries: Steppe Nomads have a very limited number of clans they can support, and each clan vassal has to be from a unique family, so instead of carrying over governorships 1:1 as vassals, the heads of great families from I:R will each be assigned a random governorship to rule over in CK2 (except for the ruler's family), with the country ruler ruling over the rest. This ensures that each clan vassal in the nomadic realm will be of a unique family, while also ensuring that there won't be too many nomadic clans, since each I:R country has between 3 and 5 great families. There will be major clan agitation initially to deal with for large steppe nomads, however, a player or AI will be able distribute lands to keep the clans in check. Another unique feature with steppe nomads is that although clans can follow any religion they want, all clans must be of the ruler's culture, otherwise the game will make that clan an independent steppe nomad; therefore, to prevent realm shattering, all characters in a nomadic realm, including the ruler, will be converted to the I:R country's culture. Finally, all nomadic provinces in a nomadic realm follow the clan chief's culture and religion, so all province culture/religion diversity from I:R will effectively be lost in converted steppe nomad realms.
Here's an example of a migratory tribe converted as a steppe nomad:
Alania is a migratory tribe with Alan culture, which is a historic steppe nomad culture, so converts as a steppe nomad under the 'Steppe Cultures Only' and 'All Migratory Tribes' options.
Alania was an above-average-sized country in this I:R save, so had 4 great families. The heads of each family (excluding the ruler's) were each assigned an I:R governorship (with the Tenare family getting a big region and Kunlyus getting fairly unlucky with a border region), with the ruler getting the rest.
If the ruler has too many lands compared to the other clans, clan agitation will effectively force a player or AI to parcel out lands to them, ensuring balance.
Additionally, even though the new Invictus 1.8 'Midas' map expansion is now fully supported, I have maintained support for the older Invictus 1.7-1.3 map by adding a Major Overhaul Mods option which changes the province mappings used in the save-game conversion, allowing you to still convert your older I:R campaigns into CK2. Currently, the Major Overhaul Mods option has 4 choices: Auto-Detect, Invictus, Invictus 1.7, and Vanilla.
