>>96176273 (OP)
Wargaming is distinct from other games in that originally the game was secondary. The game is a way to celebrate and express your creativity with this army you painted up in your own way. The game is "something to do" with the army, the creation of which was the main thing, and in reality, in a way, this is still the case. Most people spend much more time in the hobby side of things than they do playing the game
Horus Heresy was them literally stumbling ass backwards into doing this again. The narrower scope, the fact that 9 out of 10 armies were just space marines, pushed people to make an effort to differentiate and impart character to their armies. You pick a distinct legion, you work to realise that legions vibe and feel, or you make up your own chapters within the legion with their own quirks. You kitbash, pick very particular HQs or unit choices. Once again the game is the last and least important way of showcasing and playing with the sum of your creativity and work
You want a tabletop game to be dense and balanced enough that winning is satisfying, but you want that to compromise where necessary in service to being a satisfying way to express and act out your armies character, and allowing you to make sub-optimal decisions because its what your guys would do. You don't want to be incentivising world eaters players to take a bunch of forgefiends because maulerfiends, the melee option, are unplayably worse
Its not a matter of there being a single piece of advice. You need all the rulebooks and codexes and public messaging to revolve around encouraging narrative play. But GW has all kinds of incentives to do otherwise(they don't want to encourage kitbashing and unit variety like you could get with forgeworld cause its easier to streamline the range into plastic, with fewer options and higher margins due to higher production volume, and train you to want to buy that rather than than some rare, special thing or unique bits to customize your models)
