I'll chime in my two cents.
The Bad:
Doomsday Arks and, to a lesser extent, Ghost Arks are bad. S10 Large Blast sounds great until your opponent roles their cover saves and then proceeds to blow it up because it's AV13 and open-topped. If he makes you Jink then the Ark isn't firing, which is the same as being blown up. Ghost Arks used to be good until
GW FAQed Jinking transports making passengers snap shoot. I would shy away from both of those.
CCB isn't great either. All it takes is a hidden Power Fist or Melta Bomb to put it down and it's not much more punchy than a plain Overlord with Warscythe.
Annihilation Barges don't really do much. The 24" range on the Destructor really limits them.
The Maybe:
Deathmarks aren't bad, but they aren't good either. I find they are one of those high risk-high reward units. Having to Deep Strike is what holds them back. I wouldn't put them anywhere near a 1000 point list. They can be good when you get to 1500+, but you need 8-10 per unit to make an impact.
Heavy Destroyers are meh. Yes, they are our only dedicated anti-tank unit (Gauss doesn't count as dedicated anti-tank in my books), but are very expensive for what are essentially 36" range Lascannons. Only use them if you have a tank-heavy meta.
The Good:
Orikan is very strong in a unit of Sword/Shield Lychguard. He makes them nigh unkillable and they can cut their way through 3+ armour pretty easily.
Yes to Wraiths with Whip Coils.
Next Steps:
I would get one more squad of Warriors. They're a good all-around Ob Sec unit. I also love Night Scythes and would recommend getting one to move around Lychguard or Immortals or even a 15 strong Warrior unit.
As for a Spyder, the only reason you get one is to form a Canoptek Harvest. 95% of Necron players will tell you to do this and they're also going to tell you to get Tomb Blades to form a Decurion. I am the 5% that won't because every bloody Decurion/Harvest army is the exact same and it's boring. Necrons are a cool faction that, unfortunately, always end up Decurions, Wraiths and Destroyer Cults. Don't fall down the competitive rabbit hole. The
40K world doesn't need more boring Necron armies.
I hope this helps!