Base species:
Starting species will determine some starting essences.
Each species has 2 essences. First a small general effect, second often incentivises focusing on that base species for combat etc.
Either get 2 rank 1 essences or both of one species.
Team Synergies:
You get various party buffs based on your team's base species count.
Each unique species will add towards their base species count.
Enemies will also have access to these perks based on their team.
Events:
New unique world with various species etc.
A few other new events
random relax/explore date events are more likely (individual chances), but most are once/date. (early on, almost guaranteed to trigger some events, but get less likely as they diminish)

Items:
many new, mostly legendary items.
Reworked many item skills.
Most non-consumable item-skills now have uses per combat.
Items with 0 range instantly cast on self.
Skills:
Each maincharacter has a way to adapt skills of characters. (uses same one-time adapt as traits)
Druid can put a fixed set of skills onto spirits/seedlings.
Creation can adapt any character with her unlocked skills.
Evo can adapt any character with skills from the possible species/base species skillpools. Her upgrade makes it infinitely reusable
Druid can unlock various unique plant skills by exploring certain biomes.
QoL:
Quickmanager has more options. Can also shift click tabs to sort.
Can also select characters to enter breed/fuse screen with them selected. (new drop field)
Added small summary to start-game button showing starting essences/element etc.
Base species more color coded in various places.
Other:
Added new modifer where you get characters of random quality each day, but other methods of aquiring them are much worse. (can get otherwise rare/late-game characters early on etc.)

Updated some species/item art.
On the overworld, enemy party info keeps one unit unknown.
Changed order of some damage calculation (sacrificed hp shouldn't be affected by vulnerability etc.)
Temporary characters for events immediately join battles, regardless of party size.
After reaching Avy's world, she gets a different starting option in future runs. (skipping most pet training)
Many smaller changes
Noteworthy bugfixes:
Mushroom evolution was bugged
Could lose dropped items when updating overworld inventory.
Life loss effects were affected by lust/vulnerable etc. ignoring requirements of having enough hp etc.
many more
v19.0.1
Bugfixes:
Ryxhora event triggered daily
Resurrection effects caused some issues
Arena service affected all battles
overworld heal could heal less than expected
Crea corruption
Enemy overworld parties didn't move when resting on portal tiles
loadout sizes weren't saved
loadout difficulty didn't update immediately
v19.0.0
Overworld:
world changes a bit daily (forests grow/expand etc.)
some objects/encounters/events etc will be replaced/removed each day
hives don't appear in stabilized portals anymore
new structures with various effects.
workforce as a new resource which lets you just use gold to make some orders.
get more from reputation/kingsquests or firing high relation characters.
Druid starts with more and higher basic increase
new special biome
Special biomes worked incorrectly and generally only generated max 1 per portal
Can send items/resources on floor home without leaving. (from portal/camp tiles)
Some more special stuff in Avy's home world.
Characters:
New spirit (5 new species. Can't be chosen as starter)
2 new uniques
3 new other species
new skills etc.
Other:
Added limited rerolls per date. (bonus for each unplayable card)
Added more pictures to dates (mostly Hebi/Devy)
Added other pictures to gallery (many exclusive to it)
Elites are only limited to one per species (testing for now)
much higher recruit chance if only one enemy left.
Additional early common essence portal at day 2 to get started
Brothel-score regarding tips has diminishing returns
Partner dating cards now generate partially based on current lust value
Can sort by time since you got the character
Character selection always shows their current location
Lure now spawns a random enemy party nearby
Morta got a unique genetic trait
swamp slimes can replicate
Gene info on quick manager shows if a character is adapted
some other small changes
Noteworthy bug fixes:
firedragons couldn't evolve
celia date crashed game
anomalies could teleport into impassable terrain
some tavern quests couldn't be completed
AI ignored some type of skill requirements
In dates partner sometimes generated a play card
daily material income was kept over when creating a new save
event tiles in special portals didn't renew daily
slime kept spreading after hive removal
Game is locked to one instance now (might prevent some weird bugs where it opened multiple times and deleted saves.)
Only unlockables.sav is compatible