That video, a page or 2 back would be cool game play wise, now add these features and you have a great game
Here are the races:
Tyranids (as special self-customizing spawn, weird but not unbelievable)
(Necron- I don't see how they could join, without special fluff)
Here are the factions: Everyone fights everyone
Tau Empire (and Kroot)
The player starts (in most races) as 1 of 3 classes Fighter, Psyker or Tech.
Psykers have to get a license in the Imperium.
-There should be a special % of players created as blanks, omegas, mutants, and uncontrollable stuff like that. Advancement is partly based on class(title) but some skills are optional. So lets say Chaos Space Marine with a specialty in "Raptor jet-packs" and team buffs= Raptor squad leader.
Players are rewarded for number of kills.
Holding strategic points.
And killing NPCs (which stay dead as long as a faction has that strategic point)
Humans: Player picks a planet to start on. That planet has a Space Marine Chapter on it and Imperial Guard/ Imperial Navy/ Assassins Temple/ etc; you train, level and test to reach certain titles (classes).
Space Marine is achieved by finishing a forth of the training in the Imperial Guard and taking "the Exam" (I forget the name) around lvl 15. But you start as a scout, and move your way up. Upon reaching lvl cap you have the option to return a Dreadnought the next time you die, around lvl 145 (but you can't undo it). And raise the last 5 lvls, to 150.
Imperial Guardsmen come with better buffs, and group skills, moving up in lvl and rank like the real military (corporal, sergeant, captain... commander, etc.). Commissar are high lvl buff skilled fighters. Tech priests are Tech players and can build on the battle field.
Assassin: Half way through training you can switch to one of the assassin temples.
I think the game should have space combat... So Tech are best suited for the Imperial Navy. They can command, transport and attack with ships (so lets say the Imperium could block the Tau support routes but not attack). In this way Space marines can be dropped on a planet with (Tech class specific ships). Advancing on a ship is like being in the Guard... move up from shooting torrents to commanding a battle ships that deside the fate of 100 other players on board.
The Sisters of Battle are treated much like the Space Marines... just sexist.
With some sort of pets...
Chaos: Planets have certain Chaos threat level, (as Chaos can be banished and summoned) the player has to seek out Chaos. Based on their deeds and how they accomplish missions a Chaos God grants the player points. So they have to find the god specific alter on they're planet (won't always have all of them).
So scouts become cultists... etc. Mutations and Obliterator titles are given by completing planet specific missions and number of kills.
Daemons are pets, and summon-able NPCs. (Or not... I don't really care)
Orks grow in size and in title, they make strong fighters, but weak psykers, as they need other Orks to gain WAAAGH.
Tech class Orcs can become operators (machine specific) or Painboys, or somewhere in the middle.
They have a shorter advance system yet its harder to climb as winning fights with a red life bar is the best way to advance.
Other green-skin races (like gretchin) are pets.
Tau come in male and female. A way more role specific race, with no psyker class/ but a Ethereal specific class, which is like super buff and can also befriend smaller alien races and have them fight with you, based on a Faction vs Environment system (so they dislike your faction 1 more point per every 1000 of them you kill). The Ethereals give offerings and attack alike enemies like the Imperium to befriend these races.
From the Tau Empire the player can either be Tau or Kroot, Kroot come in just fighter, but can evolve in player chosen ways based on the creatures corpses they eat.
Fire caste is fighter
Air and Earth caste is Tech (you have to pick)
Celestial is Ethereal
Battle or Shield droid pets.
Eldar have no psyker class because they're all psykers, so the player advances based on what else they want to be. Fighter-psyker= guardian. Tech-psyker= ??. Super-psyker= Seer.
Or maybe no classes at all. For this race leveling up is completely free based on training call paths. Harlequins are an unlock-able secret around level 30. Tech players can't control Ulthwe.
Dark Eldar are another race of no psykers but they makeup for it, like Tau with another class; the slaver. The Slaver class like the Ethereal is PvE based, were if Dark eldar are spotted the environment will kill them on sight(negative) or run in terror(positive). Slavers make other smaller races and minor NPCs into pets (slaves).
Paths like Eldar and Imperials ...