The Ruinous Powers of Chaos, in an attempt to prevent the coming of the Imperium and order upon the galaxy, somehow spirited the children away through the Warp, however, leaving them scattered across the galaxy. Of all the 20 primarchs, only 18 were ever recorded (19, if you count Omegon). The other two primarchs were assumed either never found, or something had happened so that both primarch, and their legions' existences were erased from history after the Horus Heresy.
This gets me thinking: what if the last two Primarchs were found / not erased? Ambiguity always intrigues me in the possibilities available to explore. So I want to ask: If the two lost Primarchs were found / not erased and given their own, playable space marines, what should they be like?
How should the Primarchs from both legion be like? (Preferable with a summary of their background
What would form the culture of the Primarchs and their Space Marines (e.g. Flame based Salamanders, Viking/wolf based Space Wolves, Mongolia based White Scars, Greco-Roman based Ultramarines)?
What would be their combat doctrine? And what special weapons should they have?
Which side would they take during the Heresy and how would the Primarchs be removed from history like the others?
What would their history be after the Heresy (Bonus: if assuming they did canonically existed at one point, why all traces of them are removed from existence)
Which genders (

Anyone got any ideas?