Here’s my answer:

Tau: have them advance there tech, close to what humanity had during the DAOT. A way you could go about this is by having them reverse engineer Men of Iron from either an STC or an actual A.I that’s somehow managed to avoid detection.

Orks: have Ghazkull reach the level of the “Beast”. This would allow Ghaz to go toe to toe with Primarchs and from a militaristic standpoint compete with the top tiers in the setting. Attack Moons are a total vibe after all.

Eldar: just allow the Ynnari to awaken Ynead. If you don’t wanna get rid of Slaanesh just make it so that it’s an extended conflict between the two gods. Rather than Slaanesh rolling over and dying have her finally have a worthy challenger for the dominion of Eldar souls. This could free Eldar from the curse of Slaanesh allowing them to use their psyker powers to the full extent of their abilities. It would also give Slaanesh a bigger role in the setting.

Necrons: have the Silent King be the god tier dualists and strategists he’s ment to be. The Pariah Nexus is probably a great campaign to show this off. Other than that from a militaristic standpoint I think the Necrons are just fine.

Tyranids: in theory the Tyranid should be just fine. They don’t need dualist on the level of Primarchs there endless hordes can deal with them just fine. However something I’d like to see is the Tyranid actually winning major battles. We got a taste of this with Octarious. Just more of that please. They don’t need to take over the galaxy but holy fuck surely there’s a middle ground between punching bag and tanking over the whole galaxy.