Order gods.
>Opposite warp presence of the chaos gods. Are much weaker of course but not prone to chaotic nonsense. Work to instill sterile robotic obedience and determinism in the galaxy.
40kven.
>Actually rule the vast majority of the webway and are invading realspace and are constantly working to subsume the warp into their own pantheon.
Space lizardmen.
>Live in a few absurdly sized Dyson sphere temples out in deep space after their great old one masters disappeared and they are only now coming back to sieze control.
Bump of desperation.
>Paymaster Guilds
Tiny empires of rich planetary lords and such that have broken away from the Imperium, with armies made up of the finest mercenaries, xenos and human alike, that money/slaves/natural resources can buy. The Imperium wipes them out on a regular basis, but they're really more of a nuisance than anything else, so when there's more pressing threats to deal with they mostly leave them be, even allowing Rogue Traders to deal with them.