Fair enough. We have an opportunity to raise up the Xenos here But perhaps we can expand the detachment idea to other factions to make each one a bit more flavorful. So that the same army doesn't have to be the same army everytime. Similar to the fleet wide advantages in FTL, or the team wide advantages in Deathball.
Back in the 90s: Elves had different craftworlds (I don't know the worlds very well) Orks had different clans (Bad Moons, Evil Sunz, Goff, etc.) Planetary Guard had different regiments (Tallarn, Mordian, Cadian etc.) Havoc Battle Brothers have their detachments (Thousand Sons, Iron Warriors etc.) Battle Brothers have their detachments
You could make some kind of "army wide customization" perhaps available to all armies that costs a little bit but affects the whole army? A shooty one, a H2H one, a morale one, a movement one, an armor one, etc.). How does that sound?