It is utterly impossible to build a 40k army that goes against the background history. The most cheesy, beardy, competitive list is always condoned by the fluff. Granted, if you have Ultramarines from the 14:th company you’re doing something wrong. But that’s a painting/modelling problem, not an army building problem.
To use my wargamer psychographs, there is a false dichotomy between Ringo (fluff lover) and Paul (competitive). Some of the Ringos out there mistake a caricature army for a fluffy army. A caricature that exaggerates any details that sets their army apart from others of its type.
A quick google for an example results in this post. A good post, by any reckoning, it gives sound advice how to build a fluffy Raven Guard army. But how does the author go about it? By imposing restrictions that aren’t there. The Raven Guard still consists of mostly tactical marines. So a list with a Master of the Forge leading four tactical squads and a vindicator is just as fluffy as Kayvaan Shrike with lots of assault marines. Why? Because they exist. Typical composition doesn’t automatically equal fluffy composition.
The reason for Codex: Craftworld Eldar being assimilated into Codex: Eldar in 4:th edition? All the different craftworlds could be represented using a single codex. Same reason the Chaos Space Marine legions are gone, as Gav Thorpe explains here. Restricting or rewarding a variant army to make certain choices actually destroys the emaginative and creative aspect of both list building and background history. Without the openness we end up with cookie cutter armies. Not fluffier ones. Like I said, the dichotomy between a fluffy and a competitive list is false. Why? Because the codexes are designed open-ended. Any composition you can come up with is legal both rules-wise and background-wise. Remember that the models come first. The rules and background are there to let you play with cool toy soldiers.
Things like Salamanders without flamers. Saim-Hann with Dark Reapers. Iron Warriors without Obliterators (the heresy!). All perfectly acceptable according to the background – you don’t even have to adapt or bend it. Unfluffy doesn’t exist.
Here’s a question for you: What is special about the army selection in the following armies?
- Blood Angels
- Dark Angels
- Crimson Fists