New Legion player looking for advice on lists and purchases

It's a good starting point. One of the problems with the new theme list-focused meta (as if there weren't enough already) is that general collections like this are stuck either not playing in theme, or needing to purchase into theme. It almost seems like it might be better now to buy into a faction with a theme in mind.

Your Oracles list looks like a good starting point. That's not a bad thagrosh1 list by any stretch. My suggestion would be to run one of those carniveans as a Ravagore, however, because you'll run into fury problems with too many carniveans all in one go, whereas a ravagore will typically just need 2 fury every turn and can be handled by the shepherds alone. When you ultimately do decide to have it pull weight in melee, it can be given draconic blessing to hit hard anyways. It doesn't benefit from the durability-skew like typhon and Carniveans do, but it also can stay more safely away until it's ready as well, so it's still a good attrition piece.

Abby2 can literally be run in Oracles as a scythean/carnivean spam with a seraph (you need the seraph even with the feat). A neraph or 2 can be added to this for meaningful impact. You don't -need- hellmouths for her specifically. A screen is nice, but I find that a good abby2 list should either have a huge ranged advantage or an alpha strike advantage, and in either case a screen isn't as essential. I strongly suggest picking up Proteus, however. Or even just finding the conversion kit and using one of your existing magnetized chassis. You might even consider breaking down your fixed carnivean and turning it into Proteus. I can think of no list that would want 3 carniveans but wouldn't consider something else over it.

Purchasing suggestions:
- Proteus. An excellent beast in general, but if you want to play Abby2, it's all the bigger a deal.
- A succubus. It's not too expensive $$-wise, and it's a candidate for a free model in both Oracles and Children. Most casters benefit from one.
- A sorceress. It's not only for the theme lists, really. Legion emphasizes its flyers a bit more in mk3 than mk2 with stuff like buffing the soldier, the neraph, the speed drop on the carnivean chassis, etc. We just take more flyers now, and free charges on those flyers is a pretty big deal. That she is free in oracles is extra money, because she's a good solo overall, but even if you don't want to fork out for 2 of them, one is probably a good idea.
- Melee Units. Hex Hunters for Oracles, or Nyss Swordsmen for CoTD or out of theme, or hellmouths for out of theme. They aren't essential in any case, but they're all good somewhere.
- Battlebox. A second neraph is good to have, kryssa is... worth playing around with. Bolt thrower is good to have. It's basically the price of 2 bolt throwers, so it's an economic buy, even if you never touch Kryssa.

So if you bought, for example, the battlebox, a succubus, and a hellion, you could do this and either use the carnivean or if you get the proteus kit, that instead:

Legion Army - 75 / 75 points
[Theme] Oracles of Annihilation

(Absylonia 2) Absylonia, Daughter of Everblight [+28]
- Carnivean [19](or Proteus)
- Nephilim Bloodseer [8]
- Neraph [12]
- Neraph [12]
- Scythean [18]
- Scythean [18]
- Seraph [14]
- Succubus [0]
Blighted Nyss Sorceress & Hellion [0]
Spell Martyr [1]
Spell Martyr [1]
The Forsaken [0]
The Forsaken [0]