So what would have happened if the Horus Heresy didn't happen

We would still have a couple of primarches less due to the mini-heresy, so the Imperium would have been weaker already. After the wars of the Beast, almost every planet in the Imperium would have some sort of ork infestation, but they would be somewhat manageable, thanks to the Mechanicum not getting divided into loyalists and dark mechanicum, and thus, no more lost tech. There wouldn't be an Echlessiarchy, because most of the primarches are still there to uphold the Imperial Truth. Inquisitors would be way less important, and they'd probably be replaced by the Deathwatch.

In terms of xenos, the Orks would be the major threat, and thus, warbosses are somewhat more organized and dangerous. Tyranids would either have a hard time entering the galaxy (if orks have teliporter tech, all the orks in the galaxy could appear to have a good ol' scrap with the bugs), or it would be a walk in the park (so much ork biomass they become unstoppable fast). Necrons would try and fight the orks, but they are just too many, and many tomb worlds just don't wake up to avoid pointless losses. Eldar use buopurging techniques to kill the orks from a distance. Tau can fight better against orks than chaos, so they could have an advantage, but it would be pointless since the Imperium is stronger here, and thus, their war wouldn't end in a stalemate. The Imperium either exterminates them or turn them into subjects (some primarches and deathwatch might be fascinated by their tech, thus keeping them around to manufacture them in secret).

>>54761151
I forgot to mention, even in a weakened state after the mini-heresy, the Imperium has more manpower and more primarches, so the War of the Beast fares a lot better for the Imperium, leaving them in a stronger position than in real 40k.

>>54761151
Ok but if there isn't a eye of terror what does that say for Slannesh? How does the Eye of Orkiness affect things?

>>54761284
I think there is an Eye of Terror. Of sorts. Slannesh was born and thus, an Eye of Terror appeared, full to the brink with daemons (not that many spacemarines and chaos cultists because the Heresy was tiny and well handled). But the orks saw it, sent billions of orks to fight because "good scrap" and their waaaagh energy ended up turning the Eye of Terror into an Eye of Orkiness.

>>54761151
The tau would likely not exist since T'au would have become an imperial world were it not for a warp storm

>>54761324
Warpstorms can appear even without Chaos being a major player.

>>54761322
So does this mean we have Ork deamons? I mean, it's a giant spiral of Ork energy.

>>54761329
They'd probably be less likely, or more depending on if Gork and Mork have a lot of control over the warp

>>54761340
Probaby,but these ones wouldn't need to be connected to the Warp, they could need high amounts of Waaaaagh energy to stay coporeal. Th Eye would be filled with Weirdboys, though.