you have no idea....

I seriously hope they devote more time to other areas and plots possibilities than the tyranids as they hammered that to death in DoW 2 pretty well. Not that it shouldn't be covered but fighting against the Necrons for example is way up on my list of cool.

Why not a Hive world raid?
Your sent into the fringes of the hive to flay humans and arbites, go to certain checkpoints to herd the majority of the populace towards one of those big machines ( like in that novel 'Hell Forged) that swallows everything and collects humans to plug into sould suckers for battery power?

Or just rise out of the dirt to squash an Imperial Gaurd regiment that ran over tomb with a chimera and activated a scout force?

Maybe the Necrons rise up to take on some orks that landed and are looting something? All kinds of good stuff. And the possibilities for missions as necrons is as limitless as for space marines, or eldar ( both light and dark) as well as any other race in 40K.

I could see a Necron tomb world opening up at the end of a dungeon or an instance you have to do when you travel to a new planet. Like you are almost done with the boss battle/ escort mission and then you have to fight them off/escape. I think it could really set the tone for the planets mission type for future ones to come. Almost like a preview for what the player has to expect! : ]

I think necrons should be a limited enemy. Maybe one of the huge PvP dungeons at end level. Fight the Night Bringer for epic gear!!!

Lol, take his scythe! Kill thousands with a single swipe.
....
BECOME MORTARION

That guy has nothing on Problem Sleuth's scythe, given as a boon from Death himself.

The Blood Angels will destroy such filth.