Farseer on foot: generally speaking, in this edition, the spear upgrade is not worth it for a foot Farseer. Witchblade is a fine way to equip him. In terms of spells, always take Doom - this powerful spell is one of the things the Craftworlds codex is balanced around. For a second spell, Executioner is a good choice for direct damage, but Fortune for increasing durability is also fine. Guide is only good if you have a big shooting unit.
Dire Avenger Squads: I like to take these as 5-elf squads to more easily fill out Battalions and to have more Exarchs. A Dire Avenger Exarch with 2 Avenger Shuriken Catapults is good. The melee weapon options on the Exarch aren't that strong due to the low attack count of the Exarch. The Shimmershield is hideously overpriced for the benefit it gives, and in 8e, an invulnerable save on a unit of light infantry that's weaker than the infantry's armor save is basically worthless. 4 Aspect Warriors + 1 Exarch with 2 catapults is the way to go.
Wave Serpent: Normally, I give these Shuriken Cannons as their job is to be mobile. If your list is lacking anti-tank, a Bright Lance turret with a Crystal Targeting Matrix may be appropriate.
Wraithlord: There are basically two schools of thought here. The first is to simply load him up with 2 heavy weapons - maybe 2 Bright Lances - and call it a day. He can stand and shoot, and if an enemy happens to draw near, charging is an option. The other strategy is to give him a sword and strive to keep him as cheap as possible as he sprints up the field to start slashing things. Giving him both heavy weapons (which make him want to stay still) and a sword (which make him want to move) is a poor investment. Flamers are overpriced - stick to Shurikens on the wrists.
Add a Warlock to this army to gain access to the Runes of Battle spells and to allow to form a Battalion. From there, you can go in any direction. Wraith units, Fire Dragons to go in the wave serpent, flyers, artillery, maybe a squad of dark reapers - many options would work.