Your 40k

[Lexington]

I think the #1 method for fixing 40K right now would be removing or altering Formations. Currently, and, sadly, intentionally, they're broken as all get-out.The easiest way to do this would be to remove any special rules and bonuses from them - they're just an alternate way of choosing your army list. No freebies, no nothin'.

I actually think the distribution of Battle Brothers is just fine - it's the rules themselves that cause problems. Limiting all special rules to affecting only models of the same Codex might help.

I'd also change the way wound allocation mechanics work. Aside from the multi-wound shenanigans (easily fixed), 5th Ed had my favorite way of distributing wounds. The current mechanic is just clunky nonsense that allows Invul+FNP+EW characters to tank a ridiculous amount of damage.

There's others, but those are some big issues and solutions that I see right now.