It's been done before, and even as recently as the Arks of Omen narrative thing toward the end of 9th. No one cared about it. Here's the one from 6th/7th editionEasy fix for all of this. Bring back allies.
I hear you at the back. The tournament autist complaining about some obscure combo or some lore inaccurate team up. That's an easy fix too.
Here's how you do it in three sentences.. And in the unlikely event GW is taking ideas from an internet nobody, you're free to steal it.
"All units under the same umbrella (imperium, xenos, chaos) can ally. Allies must be 50% or less than the game point value. Allies cannot be used in competitive tournaments."
There are edge cases. Tau should have a special rule where they can ally with any imperium force, for example. As for lore. If you really care even the slightest about the fiction, then no-one is going to choose an orc-nid alliance.

Here's the rules from the arks of omen narrative thing at the end of 9th

Yes your idea is more "open" than either, but really no one ever really gave a shit. There's a ton of fluffy narrative driven missions, campaigns, asymmetric stuff, random warp storm effects, all kinds of shit within White Dwarf, crusade books, narrative series, etc. Even the 500 worlds thing and the newly revealed maelstrom thing has a bunch of narrative shit in there... that'll never get played because no one wants to buy this stuff and even when it's freely available on waha... still no one ever bothers with it.





