Neither of these are really that plausible or very likely but here is my big question.

What would be a bigger threat to the Imperium Of Man

The Tau and Eldar races (not including Dark Eldar) make a successful and permanent alliance, causing them to work almost perfectly together. With the Eldar more in less command.

OR

Ghazgull finishes his waaaaargh against the Nids. Leaving the Subsector his army is now a massive horde of green and veterans of the Waargh?

Neiter, the fully awakend Tomb World of the Necrons, gather under the Silent Kind.
Space Ships not bound by the warp.
And a Ctan "sleeping" on Mars.

But in this case i would say Eldar and Tau, they can work together, Orks will at somepoint splinter off and fight each other