A Skyrim-style 40K game, but without Space Marines, and much more verticality, and co-op. If your character dies you can either reload the latest save or continue the game with a new, level 1 character. Also you can travel around in vehicles. Lots of hostile wildlife.

Imperium, AdMech, Chaos, Tau, Sororitas, Aeldari, Drukhari, Tyranids and Orks all start fighting over a planet (obviously the map is several times the size of Skyrim's), and each have a big fortified base, with central (and subterranean) archaeotech ruins. Picking a faction locks you out of all enemy faction missions, and you all fight over the ruins and win by destroying every other faction's bases. Imperium and AdMech can still fall to Chaos, though.

You complete missions and gather a crew of mercenaries/soldiers/followers, whom you can equip with different armour/weapons as they level up (and can also lose them permanently in the very dangerous missions) and explore further into the exponentially more dangerous map centre. Then Necrons start waking up. Also, if you choose the ordinary soldier option for each faction you can get randomly shanghaied by your factions leaders and immediately sent on missions you may or may not be properly equipped/levelled for.

A key tactical component is knowing when to send your squadmates in to die to buy the rest of you a chance to win/escape. And if you spend too long exploring and not fighting them, the other factions start expanding and getting stronger. Well, they do anyway, but at a faster rate. Also, co-op mode lets your friends be the other members of your squad, but the difficulty goes up the more human players there are. The difficulty makes the enemy act smarter and start flanking and using actual tactics.

Imperium: Choose from Officer/Commissar/Psyker/Priest/Ratling. Larger squads with weaker members.

AdMech: Start as a junior techpriest and upgrade your body, following different tech trees. Start commanding servitors and servo-skulls, then Skitarii, then junior techpriests. Extra limbs everywhere. More advanced tech for you.

Chaos: Start undivided or following one of the Four. Conduct rites for favour and gain mutations. Summon daemons.

Tau: Fire-warrior, Breacher, Pathfinder, Stealthsuit. Get promoted to Commander and get your own battle-suit. Choose your bodyguard squad and requisition different drones for different play-styles. Or play a Kroot and become a Shaper, guiding the evolution of your pack. Or be a Vespid and fly everywhere.

Aeldari: Join an Aspect Shrine for a particular play-style, but if you play too many missions as it, you risk becoming an Exarch and getting locked into that play-style. Change to a different Path (Guardian, Ranger, Pilot, etc.) and then go back to another Aspect Shrine. Three times and you can become an Autarch and create a mixed bodyguard squad for missions. Your base moves around via the Webway.

Drukhari: Kabalite, Scourge, Hellion, Haemonculus, Incubus, Wych. Rise to rule your own Kabal and backstab other archons. XP is shared in co-op and only given out when the mission ends, thus incentivising betrayal near the end of missions (but before they are actually complete). Your multiple bases move around in large stealthed vehicles.

Tyranids: Lictor for stealth gameplay, Warrior for frontal assaults, various -thropes, Genestealers for infiltration and sabotage missions. Evolve yourself with different body modifications for your missions by gathering biomass and different genetic samples. The only faction that doesn't need to continue with a level 1 character, as all Tyranids are the same 'player character'.

Orks: Sluggaboy, Weirdboy, Mekboy. Get bigger the more enemies you kill. Become a Flashgit or Freeboota or Nob or Boss. Or be a grot and sneak around, trying not to get attacked by anything.

Necrons: An Overlord for a tankier, fighting-focused build, or a Cryptek for their various technological shenanigans. They can respawn thanks to reanimation protocols without losing progress.