A few buddies and I got writing some crunchy bits on this, and we had a 'fuck it' moment.
We decided, with all the 'unknown sector/segmentum obscurus' stuff flying around, we'd just go non-codex.
Here's what we came up with: Marine list gets gimped. No tanks, no jump infantry, termies restricted, landspeeders modified.
Guard-type unites added as troops. Restricted versions of infantry battalions (pretty much conscripts) are a troop choice. Some guard-type tanks and artillery allowed as support.
Also, focus put on cavalry, namely mounted marines.
Army setup goes pretty much: Small, elite, fast mounted marines, some marines troops behind them, and the mass of guardsmen following behind them. Some guard-type support behind that.
The structure goes: Mounted generals/damiyo, Armored samurai, and the masses of peasants that made up the army behind them.
The fluff behind this: The original chapter was sent to a small, obscure, system that hadn't been paying tithes for years. Upon arriving, they find a heretical system that stretched down from the governor to top millitary officials. The chapter master basically went "fuck this, I'm taking over," and then he and his chapter set about entrenching themselves in the system.
Each company rules a world (or so,) with a hereditary system flowing down bloodlines where the eldest son is tested to be a Marine, becomes a marine, his younger sibling carries the family name and honor, and then, his eldest son takes the tests. For company commanders, they maintain political control over their families while marines, but day-to-day politics are controlled by their younger sibling.
It's a pretty great departure from what this started as, and it needs an ass-ton of help to get fleshed out, but, I think, it could be non-sparkle-snowflake, balanced, and far more interesting than another codex marines reskin.