There has been talk of new factions in 40K for a while as it has been a while since one has ben introduced.
I thought it maybe fun for people to give ideas as to what could be next either based on lore and cannon or just your own immagination / wishes.
My Idea would be given that a primarch has returned and has seen what has happened to the imperium and how it has been bastardised from its intended purpose to the now "imperial cult". To set the scene we see two more prmarchs returning because they are my favourite (in order) and because i feel their personalities would lend credance to the narrative, Leman Russ and Lionel "the lion" Johnson. Both Primarchs would share Guillimans disdain for the Eclisarchy but unlike thei brother they would not be unwilling to eradicate it. Now Guilliman has his brothers by his side he would feel more confident that the sight of three primarchs saying the same thing about how the Eclisiarchy and the inquisition being the opposite of what the emperor would want.
While there is 3 primarchs denouncing the cult this would lead A massive proportion of the imperium to do the same but others would be more reluctant. When eventually the trio of primarchs release the information, buried for melenia, that the Cult was based on a book written by Lorgar. More imperial citizens abandon the cult revearing the Emperor regaurding him as their lord and savour but NOT a god.
BUt pockets of citizens on worlds across the imperium stay loyal to the heretcal Cult and with the eclisiarchy leave the imperium with parts of all Adepyus deprtments joning them all save for Astartes with the gene line from the 3 Primarchs.
This cult still believeing them selves loyal to the Emperor but at odds to who he is to the imperium as a whoe will fight the imperium for what they believe in but will also fight heretic and xeno alike there will be times they may side with any of the other faction in order to defeat a greater enemy force.
If you have any other ideas for factions please share them here.