It wouldn't be sad if it simply wasn't vanilla marines. The Legion of the Damned, Black Dragons, and Space Sharks are far more interesting and curious than Chapters like the Ultramarines and the likes. Hell, one company of the Legion of the Damned wiped out an entire Blood Crusade, killing millions of warps-spawn, Daemons, and hundreds-thousands of Chaos Space Marines, as they covered the entire planet in the body of CSM's in service of Khorne. They're pretty awesome, although being invincible specters of death helps.
They likely wouldn't work. They're far too squishy, even more than Eldar. Not even Stormtroopers would stack up to any of the other factions very well, save maybe Ork Boys. They'd be better off adding something like the Tau, as the weaker Battlesuits and Stealthsuits would fit in nicely. Same reason why adding something like the Necrons wouldn't work either- to be remotely close to lore- they'd be hella overpowered with Gauss Weaponry and the likes.
Eldar primarily. But whenever I get tired of being tactical and sneaky i'll likely go on a rampage with the other races.
