There were significant changes to damage and XP mechanics in 0.026. After playing for a while with a saved game that it is playing through the Story, I wanted to provide some feedback. Obviously, these are just my personal opinions.
First, the XP changes are excellent. Prior to 0.026, leveling up a new creature really felt like a chore. In particular, it felt like the only reasonably efficient way to level up a new creature was to run realms with the auto-defeat enemy god-power (I forget what that item is called, maybe Heart of Darkness?). Now it feels like running higher level Realms is appropriately rewarding and new creatures reach a useful level in a reasonable amount of time. I really think the XP changes for low-level creatures were beneficial.
Second, I think the newest damage changes make one-shotting enemies a bit too common. Personally, I enjoyed the 0.023 era damage formula more than the current 0.026 era damage formula (One-shotting was possible in that iteration, but not particularly common). If you are gong to stay with something similar to 0.026, I would seriously consider either making Defense more powerful/impactful, or else just significantly increasing the health of everything in the game.