Especially since splitting the IoM into two or more rival factions rather than allied factions will actually help balance their population better. After all, part of the reason SMs have 40% right now is that many Imperial players will default to Space Marines if their preferred Imperial faction is not in the game. Adding the other factions will draw these players away, making the Space Marine population more balanced.
It's looking like combining SoB and IG as a separate faction is the best option. No clue how they'll explain it but I'm sure they'd come up with something in that scenario.
Why is that? The both have more than enough unit types to flesh out a faction easily and it's, if anything, easier to justify them fighting each other than it is to justify them working together directly.
IG vs SoB vs Space Marines makes no sense. Combining them still makes no sense but isn't quite as ridiculous. IG has a variety of troops but would have to rely on numbers. Combining them could allow IG to be another F2P in the future with Sisters and higher ranking IG as incentive to buy the faction. It rubs me the wrong way entirely but they're too big to not include eventually. I'm assuming it will take some time though. Tau will probably be the first new faction.
SoB are easily a stand alone faction, they're an Inquisition faction and Xeno hunters, IG is also easily a stand alone faction, yeah they die easily but they will have low requisition costs due to being squishy and they still can pack a huge punch with their heavy weapons, tanks ect. In fact if IG come into it as a single faction then being that they die so easily compared to space marines they would be the only faction that it would be balanced to include assassins and GKs in, the OP of a few few units will be offset by the squishyness of the rest.
Um, I thought they are the militant wing of the Ecclesiarchy.. which is not the Inquisition... They have some overlapping duties but are not connected.
Either way it doesn't make much sense with the lore, but at least with them all being separate factions it can lead to good gameplay. IG should definitely be squishy/numerous, but there are plenty of ways to represent them that would have them be a fearsome faction while still representing their frailty. Lower requisition costs. Lower spawn times. Stronger vehicles. Passive requisition generation. I fully expect there to be a 'vehicles' expansion at some point in the future. IG could easily be the poster boys for that.
This could make sense if say the Sisters of Battle are lied to and convinced that the Space Marine legions are corrupted. Not the first time its happened in the lore...