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I reckon that it will devolve into just Space Marines.
But then people will get sick of just having marines and so different varieties of marines will be released:

Normal Marines, that are Gods
Mini-Marines, that come in hordes, and have to have loads of vehicles to survive
Zealous Marines that hate everything different (female mini-marines too)
pansy elf-like marines with better weapons and anti-gravity weapons.
Evil pansy elf-like marines that will torture you for fun
Evil Spiky Space Marines, with evil demons
Skeletal Marines, that will steal your soul
Green, tough marines, with primitive weapons
highly advanced blue marines, with loads of guns
Bug Marines, that live only to eat.

That's a good plan, I like that.