I've had quite some ideas for expansions to AoW4. For example: https://forum.paradoxplaza.com/forum/threads/my-longlist-of-suggestions-and-changes.1712630/ And: https://forum.paradoxplaza.com/forum/threads/possible-future-tomes.1720319/
However, there are some basic things that might get in my way even more when trying to make factions. For example, a tier 1 and tier 3 tome dealing with water magic (including the ability to summon tide spirits of course). Or a tier 2 and tier 4 tome dealing with cold/snow/ice/winter/hail. If those were added you could have an entire run of just water and ice tomes, at least from tier 1 to tier 4 (tier 5 tomes are a different, but also problematic, issue).
Another basic thing is tomes dealing with Warfare. There are tomes like Meditation and Horde that sort of scratch that itch. But there are very few options for a non-elemental run at the moment. You always end with Demonic magic or earth magic or engineering or what have you.
What sort of tomes do you guys feel are missing that limit the kind of fantasies you can create in this game?