First thing they need to do is have a real play-testing community. I'm not talking about the guys and gals who work in GW as their main 9-to-5 job, but a group of people, numbering thirty to fifty in size, from ages 12 to 60, that are given the pre-print versions of the book and told to play the game.
If you've ever read a battle-report from the in-studio folks, you will notice right off the bat that they do not play the game the way other people play it. They do not even play the game the way their rules are written. Their games are full of "illegal" units, "illegal" armies, mixed-up FOCs and other shenanigans. Are they "forging the narrative"? Possibly, but no one wants to pay the money these books cost for poor guidelines.
The playtesting team goes through it page by page, testing every unit, every rule, every possible way the rules can interact that they can think of, and they write down anything that the testing team finds broken, confusing, ambiguous or non-sensical. As mentioned previously ITT, one problem the rules-team has is that they wrote the rules, they already know what the RAI is. An outside perspective is required to highlight areas where RAW and RAI are unclear.
The playtest team is also tasked with finding balance issues. "Hey, so, this army here has Unit X that can do ABC, but these other three armies have to spend three times the points in these other units to have a chance at killing Unit X, but those units otherwise don't do anything" or "We get what you're going for with this rule, but the situation in which this rule will actually be useful is 1 in 1000 games. It either needs to just not exist or needs to be simplified to be more useful, as it seems to be the reason this unit exists."
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