One of the main post-game modes in SS will be exploring the otherworld, where you'll be accosted by enemies, propositioned for sex by lunatics, and happen upon the occasional treasure for free. Each "expedition" into the otherworld requires an increasing amount of G, starting at the humble 10G, but it ramps up pretty fast.
You can earn large quantities of materials for alchemy by exploring the otherworld. Fighting tough enemies tends to grant the best rewards, but S-ranking a sexual encounter with one of the weirdos therein also has a chance of granting the highest-tier of rewards.
This is mostly just a flavored grind-fest to allow players to build up massive, cheat-like stocks of materials to either take into NG+ or just to allow some alchemical experimentation in the post-game, though it does feature an attached "secret storyline" style quest (that just means it's a request without a special trait reward).
Right now there are three types of events planned: sexual encounter opportunities, fights, and just treasures, but I do want to try to add something that's a bit more like a puzzle. Maybe I'll bring back the maid academy "swap-text" style of puzzles, or maybe something totally new. I may also add a few unique events.
What other post-game content will there be? I'm not sure right now.
I do want to do some fun/non-canon requests, but those will come to a post-1.0.0 world. I also have plans for a unique, post-game only romance path (Charlotte X Priscilla X player character), but that's also sort of low priority and will probably come out in a v1.1.0 or something. I also plan to add some stuff with Rachel (nothing lewd) for a future version as well.
For v1.0.0, there's really not much else I think we need. I want to keep v1.0.0 pretty lean; NG+ will present technical and testing challenges all by itself, and I have a lot more dates I need to write as well, plus I want to polish up some of the endings a bit, particularly the girls' night epilogues.
Part of wanting to keep it lean is also because I want to release Hex v0.1.0 as soon as I can.