Should we have a blanket "No WH40k" rule here? Or should we limit any crossovers to individual characters or other small scale things?
Why would you do that? 40k is the litmus test; if you can do ok in that universe you can make it just about anywhere.
Well i've thought about it for a while. I think that Spartans would be better fighters but the Space Marines would be better soldiers.
The irony of this is brilliant. Though I doubt the historical Spartans would beat Spehss mehreens... Crossover?
Even then, I'd only give them maybe a 33% chance of success, if they had the element of surprise and got the first strike. All it would take for the Astartes to kill any of them would be one or two hits with a bolter, or a single solid hit with any heavy or melee weapon, by my estimation. So if you had the greenest Astartes you could get, with nothing but a chainsword and bolt pistol, matched against a veteran team of Spartan-IIs used to working together, well armed, supported by a trio of Smart AIs, with surprise and pre-established ambush positions... maybe.
All it'd take really is a single Spartan with a plasma sword. Don't really expect any fantastic results however unless you're talking about Masterchief or something. They are like super inquisitorial stormtroopers! Good, but not as good as a 7-8 foot future alloy giant with a semi-automatic rocket launcher.
I think you are underestimating the speed of the Spartans. They can dodge bullets at near point blank range.
How do they die?!!
Seriously, picture the consequences of the Thing actually gaining a powerbase in the galaxy. Each faction would probably be infiltrated
Imperium - purge the heretic, burn the unclean. Good luck Jeffrey.
Orks - They'd probably love it. Unless they were burnas. Burnas would set everything on fire, probably out of boredom.
D/Eldar - Doomed
Necrons - Laughing
Tyranids - Jeffrey x Murphy
Tau - For the greater assimilation!
Spehss mehreen - Would be devastating if only for the loss of gene seed
Adeptus Mechanicus - Some of their humans don't even have biomass left...
Tzeentch - As planned???
Nurgle - HELLS YEAH
Khorn - HELLS NO
Slaanesh - Nightmare fuel
-and no one would be able to trust anyone, for they may not be who they say they are. Worlds are lost daily, and when it gets ahold of pskers it gets even worse.
In other words, a normal day in 40k!
Pretty much.