Here's hoping for a book 2 with updated warscrolls and more sensible abilities. The Euphoric killers rule is great. Depravity and the hero mechanic make our army too hero focused that taking any other troop that isn't battleline to focus on heroes is then seen as bad. And if we end up wanting to move away from hero hammer, our choices currently in book are just okay, not phenominal for what we end up paying. I'd like to see a return to a more balanced force where you want to take units. Give some buff to heroes, perhaps keep locus, but making some shift to letting other units generate depravity in other ways.
Fiends are not great, you want them to be an anvil, but they don't do the job as they lack mw and ranged defence. Their attacks are weak. You want them to go after big targets for essentially medieval lascannon shots, but that takes away from our depravity. Also so get close to getting fiends in a good defensive spot, its 380 points, when you could just take a keeper for the same cost, who has a strong defensive ability in locus and can generate summoning. These models either need a boost in offensive capabilities. I would either see damage 2 base, with 6's to wound being something different and allowing fiends to be in the epicurean reveller's detachment to actually get some benefit. Or if their intention was to keep fiends swingy, again allow fiends to actually benefit (so d3 mw).
We have seekers which while fast, do hit somewhat hard with a lot of attacks, but the downside is their base size. The cav base makes them hard to get in all of their attacks, so you must spread them out, which might mean tagging more things that you wanted and then you have this incredibly large footprint that might get permastuck in not piling in, or being forced to spread out more. To alleviate this, then you have to send them in in more of a blob, which just ends up lowing their offensive capabilities. While awkward to try to get the max of euphoric killers, they actually are decent and since hellstriders lost battleline, these girls will probably do better, though a way to get them battleline would be ideal.
Daemonettes are very squishy at only a 5+ save, but do have run and charge. The 4+/4+ profile is just okay, with euphoric killers meant to effectively make it a 2+/4+, but the lack of wounds modifiers and an expensive battalion does hurt them. Going back to 100/270 would be ideal and perhaps battalion down.
Hellstriders as non-battleline seem they could've done a little more. Personally claw spears at 2 attacks with 2 damage charges at 120-130 would've been fine, rolling 6 attacks that will do little damage is just okay. Whips could've been back to the -1 to hit fest as a support unit. Let claw spears be shock cav with a good amount of attacks and let whips just be there to hit light infantry and be annoying.
Normal chariots end up I feel being taken often due to wanting to take the hero versions, though, I could see regular seeker chariots taken for god seekers, if that survive into new books.
I'm not personally married to any of this, but just spitballing some ideas to get slaanesh feeling better with us using more of the awesome models that many of us bought.