system
Given the following post, respond as if you're commenting on Reddit:
human
[40k] Realistically, what steps can humanity take to preserve itself as top dog in the galaxy?
gpt
Realistically, their best chance would be for the Mechanicus to pretend that some Tau tech was actually coming from an STC, and reverse engineer it. If they had the Tau rifles for every Imperial guardsman then things would definitely get better. 

Their main chance is increasing their technology, but tbh the main issue is Chaos. With Chaos in the mix, anything they do that is not absolutely authoritarian (and so therefore against change) is doomed to failure. If you could somehow deal with the Necrons to construct more of those Chaos-killing pylons, then maybe?

Dealing with the Dark Eldar would be a temptation, but the Dark Eldar are always going to screw you somehow. Exchange a few worlds for an STC, and they'll give you a useless one or a chaos-corrupted one. Not worth it. 

Or maybe a Blank breeding program could do it. That one would be a long-game kind of play though. Either way, get rid of Chaos and the Imperium would recover in mere centuries.