I don't really think that everyone is getting the whole idea of the Caliban situation and the Dark angels, They lived on a planet with many creatures that had been tainted or were monsterous enought that they thought they could be tainted, the planet itself wasn't some un-holy shrine to the Chaos gods. The whole fracture i the legion happened because some of them wern't allowed to come out and play with the other children.
Without the powers of chaos tainting the other legions and without them Forcing events to go like they did.
I think ultimately that the Great crusade would have just kept on going untill the forces of the imperium found a decent place to fight, ork homeworld, elder Webway maybe even the tyranid controlled systems.
They would have fought until they had conquered everything that they could find to conquer or until they found a foe they coudln't beat outright, someone/ something that would have matched the advantage that the astartes had. Most likely tyranids.
Curze and Khan would have been used against the Dark elder/ elder pirates treat. With Alpharius setting using his knowledge of hit and run attack and a vast informant network to help pinpoint the location of the attacks and where they came from.
Angron, Perturabo and Magnus would have been leading the fight against the Tyranids, with Magnus working towards a method of interupting the control nord queens and hive tyrants had over most of the tyranid hordes before angron got stuck in. Perturbo would have been developing any bases/ fortresses and ensuring that they can withstand the kind of onslaught that the Nids could throw. Omegon would have been unsing his forces to conduct hit and run attacks against the largest forces of Nids where a direct astartes attack might have sustained heavy casualties.
Sanguinius would have lead the much calmer and less crazy blood angels legion against the orks along with fulgrim and ferrus manus. Eventually almost wiping out the orks.
Roboute guilliman would have continued his work with the Codex astartes whilst still making Ultramar into a place for holy worship.
Horus, mortarion, lorgar and vulkan would have fought to cleanse the necrons from several worlds but would have found this slow going, eventually leading to the development of arrays designed to stop the Necron "Phasing", so a dead necron would remain dead, thus giving the astartes a finite amound of necrons to fight on each world.
Dorn would have spent much time improving the defenses of the core worlds of the imperium eventually going on to create a Battle moon, taking one of holy terra's moons and converting it into the fortress-monastery for the imperial fists, the battle moon would take the Imperial fists legion to where they needed to fight and contained the most powerful weapons of the imperium to be used in bombarding planets and event going as far as to destoy the necron worlds that could not be taken.
Corax and the Lion would have been the deffenders of the realm, they would have been charged with the protection of the outer worlds from whatever decided to attack, the lion would have made first contract with the Tau race.
Russ would have been the Emperah's loyal body guard, he would have been responsible for keeping the Emperah safe from attacks and (when needed) policing the other legions during disputes. More akin to the commissariate.
The Emperah...... He would have taken mankind back to the former glory it had during the "dark" age of technology. He would have been the person who saw that the Tau would make a fine addition to the Imperium and barterd a deal to let the Tau form part of the imperium in exchange for their military support the Tau would gain acces to some of the imperiums vast amount of resources (minerals and warp technology mainly) the Tau wuld be responsible from keeping the Imperium safe from their side of the galaxy.
Without the Emperah sat in the golden throne and mankind now following Science instead of religion the Imperium would eventually grow to be a vast giant of the universe, technology that is now rare would have been commonplace ( jet bikes, dreadnoughts etc) and eventually the Imperium would kick off with the remaining Elder, however after some chatting over tea and pudding they would have agreed to a truce. The elder would keep their section of space and mankind wouldn't bother them, but when one called for assistance the other would honour it and send forces.
Everyone has a purpose, just so happens mine is more of an example to others of what NOT to do.
I know it's time to go to bed when the last post in every thread has been written by me.
Hooobit, nothing more nothing less.