An additional human faction would be cool. No idea what it would look like exactly though.
A human-xeno confederation tho. Outside of rogue traders and inquisitors, renegades and what not nowadays, you won't see that in present 40k. That is at least, in Imperium.
But i was flipping pages through Fulgrim
novel the other day and I remembered the Diasporex and thought to myself, " I would like to see this idea expanded and brought to present timeline. "
As for how it would look, I had a vague idea ( I write too much fanfiction really ). Their technology would be comparable to necrons' in a way they would use a variant of gauss rifle technology and simillar magnetism propelled weaponary( used in m2 by humanity), and their troops would be consist of heavily augmented human-cyborgs.
They wouldn't have the numbers of Imperium, but their standard troops would be a good degree better than guardsmen. They wouldn't be exactly good as astartes, but something akin to thallax cohorts of 30k mechanicum. https://www.forgeworld.co.uk/en-EU/M...th-Multi-melta
but with ful consciousness and independant thought. Or you know, Men of Iron.
Xenos im not sure, but I'd like to see something like Mutons from XCOM/XCOM 2- http://vignette2.wikia.nocookie.net/...20160220225023
As big/bigger than Astartes, with emphasis on tactical melee combat instead of mindless charging like orks/daemonkin do.
Their background story would be something on the line of them being a part of expeditionary force from terra back in early 20 k set for the halo stars, in order to expand humanity's territory. They reached the very edge of the galaxy, but soon fell to predations of multiple different xenos race, especially massive ork armies.
The planets they managed to colonize were destroyed by a massive Ork Waaagh and they were forced to retreat to starships and soon became a fleet-based civilization, like nowadays Craftworld Eldar. However, navigating halo stars without avoiding combat was all but impossible, and they were soon ambushed by another massive Ork fleet. Enter " - new Xenos " out of nowhere, and actually help them defeat the orks.
A temporary alliance is formed as the race is revealed to be " - blip " and possesses a searing hatred for Orks since their planets were all but destroyed by them. The remnants are also fleet based and so the two civilizations join up together. The Xenos' contribution to the alliance would be in a form of heavy shock troops since their species specialises in brutal melee combat and have the stature and strength to maul an astartes to death. However, their tech is relatively low and is dependant mostly on projectile based weaponary as well as very slow ships that are not capable of warp travel.
So the expeditionary fleet offer them a safe " haven " amidst their fleet and introduce to the marvels of peak humanity's technology, like STC that are able to create just about anything, heavy power-armour like armour that could capitalise on xenos' resilience and power, cybernetics and what not.
But best of all, they offer them a place AMONG humanity, and promise them to make them permanent " citizens/allies " of whatever the imperium was called back then in 20k.
However , as we all know, massive warp storms occur during 25k and fall of Eldar and what not, and so the return to Terra and baseline humanity is judged to be impossible at time. However, after setting up temporary colonies on several planets, some 10k or so later, they return to " Iom " only to find it much different than it used to be. Where once progress and enligthement ruled, now there's only barbarism and extreme xenophobia. They learn about this so-called Emperor of Mankind and forces of Adeptus Astartes and decide to contact Iom and return back to their old homeland.
They try to explain the rest of humanity about their new allies and how all could benefit from their strrength, but the xenophobical imperium will have none of it and orders them to destroy their allies or be destroyed in return.
Naturally, they refuse and war ensures.
I'd like to see something like that in regard to future development. Im personally not interest in squats or anything else, as we have enough tropes already.