Hi Yanyan, I'm glad you like the collection!
Minifantasy collection is a community-driven project. The new asset packs are suggested and voted on by the Minifantasy Patreon community, so every month I make the asset pack the majority has voted for :)
I understand you wish there were more creatures and magic effects, but every developer's project requires different assets, so I imagine that the most desired next release varies a lot from developer to developer.
However, I have good news for you, the next asset pack, which will be published in a few days, is an undead creatures asset pack ;)