Now, primarily, I'm writing this for storage in the <not-big-truck> that is the internets, but if you wish to comment with plus\minus\wtf of it all and maybe even suggestions or ideas...
BUT! Here it is.

NB: Keep in mind that I'm going on my predicted effects of the new Marine Codex for the basis of this.

The Chapter of the Immortal Vanguard, the guardians of Balcora
Chapter Organisation: The Immortal Vanguard are designed with infantry tactics and strategies in mind. The continual and repeated near s of the Chapter have led to an inordinately high number of Scouts, spilling over into several other Companies within the Chapter. (1 Scout squad may be used as Troops, up to 6 Dreadnoughts).
Being surrounded by a Warp storm that twists around most of the Chapter's territory, only 1 point of entry\exit is actually safe. Also, there are many cases of a Space Marine seeming to be killed but revivifying, their soul seemingly refusing to leave their body. Such individuals are treated as best they can and assembled to be collected by the Revenant's special force. (The Company).
Seeing as the Chapter was ambushed due to one of their home worlds being a Tomb world, the reliance upon Necron-killing powers was high. (Melta weapons are REALLY common).
The doctorines of the Immortal Vanguard's infantry are to fire at range at all possible times while advancing a staggered line, while at the same time intercepting any flankers with mobile ground forces. (True grit). And the Chapter believes strongly in joining any fray their brothers are engaged in. (Counter-attack for all).
Livery: The Immortal Vanguard's 1st company has a white inset and black border on their pouldrons, then from 2nd to 9th the colours change: Starting as Silver\Silver, the border cycles down to Light blue then Dark blue, then Silver again as the inset is changed to Light blue, following this pattern. The 11th Company, the Company paints the left pouldron inset black over any colour it originally was.

Colour schemes: The Chapter has had a schism between the Chaplains and Librarians of the Chapter, which the Librarians won. The Librarians now wear the black, though they don't wear the macabre skull and bones motif.
The Apothecaries wear the skulls & bones, not painting themselves in white, but in the colour of bleached bones, and they often have half of their Power armour painted in black to show their stance in the middle of life and .
Due to short time of being able to compute, I will take my leave now and continue this later.