Lots of great ideas out there.
If the Spirits took the Cats' slot as a reserve Clan, here is what I see:
1. The Spirits with their 'hidden' Galaxies, besides turning heads in the Homeworlds, behave differently than the Jaguars on the planets assigned. Resource poor, they work with the former Combine population to increase production across the board. Done correctly, they might be able to win over the non-Japanese population who had resented the Combine.
2. The Spirits bring in their two known allies, the Mandrills and Cobras. Based on the Wolves bringing in the Horses, I see each Clan receiving two systems with, of course, a small Spirit force taking notes. The Cobras, given their religious beliefs, are more tolerant towards their conquered planets populations, as long as they behave. Assuming this is successful, the Spirits and Mandrills follow suit. At this point, the Jaguars notice and maybe follow suit as well.
3. With their small fleet, the Spirits must figure out something and contract with both the Sharks and Ravens for transport and maintenance creating a potential situation where each Clan receives a part or its own system to govern reducing Spirit garrisoning costs freeing up troops for the frontlines.
4. The Jaguars are ticked, but realize the Spirits have hit upon something sending out feelers to the rest of the Homeworld Clans. The rest of the Homeworld Clans, seeing what has happened between the Spirits, Mandrills, Cobras, Sharks, and Ravens, agitate in the Council and begin approaching the rest of the Invading Clans. Deals are hammered out (mostly out fear of being left behind or vulnerable) and suddenly every Clan has a presence in the IZ. The IZ now looks like a map of Germany from the Middle Ages.
5. That said, the Bears and maybe the Wolves do not bite as deep. Their holdings stay mostly in one piece forcing the other Clan IZ's to take the heat. At Luthien, the IS forces take hideous losses and maybe Luthien falls. Even with Wolf's Dragoons, I think the combined forces from the Jaguars, Spirits, with elements of the Cobras, Mandrills, Sharks, and Ravens would prove overwhelming unless Hanse commits more to the table.
6. Tukayid turns into a crapshoot at this stage. If Luthien had fallen, the lessons learned by the Clans there would be discerned by the rest of the Clans. End result, I doubt the offer from Comstar for a proxy battle at Tukayid would be made. Instead, Comstar redoubles its efforts to boost warship production throughout the IS and at Terra. Planetary bombardments of troops become regular, along with Jumpship and shipyard destruction.
7. Eventually, the IS sends out a last chance task force which makes its way to the Clan Homeworlds and begins nuking each one in turn until the task force is destroyed. At word of this, Clan Warships in the IS begin the indiscriminate bombarding of major population centers everywhere they can, even hitting FWL, CC, and FS planets.
8. Billions are now dead. Countless economies are ruined. Knowledge across the board is lost. Welcome back to the early days when a small Jumpship with only a Union-class Dropship could conquer a planet.
