"All that being said I like having my own color schemes and fluff over using the stuff GW designed. I tend to like a certain narrative theme and GW's stuff doesn't really strike the proper balance between being interesting, tragic, awesome, and not a giant pile of fan service gak. It also gives me the ability to be creative and flesh out ideas without being burdened by what others have done or feeling like my army's narrative is stepping on the toes of somebody else's fandom.

For example I am working on making a custom space marine chapter called the Void Krakens.
Image
Fluff: Basically the Void Krakens are a "chapter" of marines that were lost in the warp at some point around there heresy or shortly afterwards. They awake with no knowledge of their history, names, mission, or even what the date is. The inside of their Battle Barge was stripped of all legion iconography and the only thing they find is the XIII but every one has either the paint scraped off or covering up the II so only the XI is clearly visible. Words like "Traitor", "Heretic", "Mutant", "Corrupted", and "Purged" are found carved into the walls throughout the Battle Barge. Having been missing for thousands of years all record of them has been lost and no memory of them (be it part of one of the great legions or a successor chapter) remains. While they don't appear to have fallen to chaos, they exist as a rogue chapter as their mere existence is unrecorded and thus an automatic red flag for any imperial official or inquisitor that goes snooping around for information. They continue to serve the Emperor and fight the enemies of mankind by arriving on the battlefield to deliver a quick and decisive blow to the enemy and then quickly departing before their "allies" gratitude for the assistance turns to suspicion. They take the name from the Void Krakens that lurk in space preying on ships as they themselves survive off raiding (usually) derelict ships for supplies. Without legitimate access to Imperium logistics they have become increasingly renegade like in their attempts to survive and keep fighting for the Emperor.

Basically the idea is they are a tragic "chapter" that through (probably Tzeentch) warp shenanigans are put into a position where they can't properly rejoin the Imperium due to the IoM's massive paranoia about heresy and chaos corruption. They struggle to keep fighting for the Emperor they love but they must resort to more desperate acts to survive. Notably piracy, stealing from the dead bodies of other chapters, and abducting people to get recruits for new space marines. In a normal society they would of been able to rejoin after simply explaining what happened but the current IoM is too far gone to let reason prevail and instead its paranoia that dictates that they are heretics. The seeds of doubt have been planted in them and as they continue to struggle to serve a IoM that would hunt them down. They start to struggle with themselves and if they are truly loyal to the Emperor or if they should bother continuing the fighting the fight like this. All the while Tzeentch is probably smiling as he watches this unfold.

"- Awakened from some sort of stasis sleep these astartes discovered that their strike cruiser is floating adrift in space. All ship logs are missing or heavily fragmented while the memories of all aboard are clouded. All the power armor has been stripped of paint and the few remaining markings show a XIII but with the XI being mostly scratched out. Unsure of who, when, and where they are the space marines quickly begin working to salvage resources and raid wrecks of ships to find both equipment and clues as to what Is going on. The astartes decided to call themselves Void Krakens after the mysterious space creature who also feeds upon ships lost in space."

"Basically a chapter strength worth of marines from before the Horus Heresy became lost in time during warp travel and eventually exited the warp in the "present". Having a collective amnesia and all records on their vessels destroyed, they only have the iconography of the Kraken and the 11th Legion markings to go by. They take up the name of the Space Krakens and continue the fight against the enemies of the Imperium. While their loyalty remains with the Emperor, they quickly learn that the 11th Legion was purged from all Imperial records and it is unsafe for them to make themselves known to the Imperium at large. The limited interactions that the Space Krakens have had with some backwater Imperial worlds (before learning about the current state of the Imperium) have drawn the attention of the Inquisition who heard rumors of this group of unknown Astartes with its alleged 11th Legion markings. They operate as a renegade chapter but maintain their loyalty to the Emperor despite being hunted by the Imperium.

I am toying with the idea of having a gene seed mutation that in a small number of them causes Cthulhu like tentacles to grow from their face and eventually lead to insanity. The event of them being lost in the warp most likely have some sort of connection to chaos but it is unknown what was done or why it happened."