I am completely on board with joining this. Either as a rewrite of the Heresy with custom legions or secession. 

I have to admit that after reading some of the [url=https://1d4chan.org/wiki/Alternate_Heresy]alternative heresy[/url] that have been done in the past, I did come up with an interesting idea for a chapter somewhat inspired by one of the primarchs of the Hektor Heresy and the cannon lore of Warhammer 40k. 

I won't go into details here but the basic idea is for the Primarch in question to land on a world like Ibsen which has a human and Eldar population that actually gets along pretty well. Likely Exodite Eldar. The end result being a Primarch who is sympathtic to xeno's races and believes that rather then wiping out all Xenos completely, those who are willing or have proven to be able to co-exist with humans in a relationship that isn't master/slave should be given the option to not only exist, but be allowed to become apart of the Imperium.