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What is lacking in this collection? 3 Tomb Blades, 10 Deathmarks, 10 tesla Immortals, 10 Guass Immortals, 44 Necron Warriors, 3 Destroyers,
Triarch Stalker, Ghost Ark, Annihilation Barge, 2 Overlords and 1 Cryptek.

I would say wraiths (I personally really like how they look and they do alright on the table)or maybe some more tomb blades. If you're trying to be a bit competitive I would wait for the codex to drop and see what's good.

I'd say wraiths too, along with maybe another cryptek, just to round out a battalion if you end up not liking overlords (or if they bring back the pseudo-psyker cryptek idea)

As already stated you definitely need some wraiths. Another box of destroyers to get you up to a unit of 6 would be really good, too.

Wraiths will be overpriced in the upcoming incarnation of the codex. But with one unit of 6 you cannot fail.
I'd add 3 more Tomb Blades.