I like the Idea but given that every mod that where tis is going to be a problam probably means that you will miss out on impactfull gameply eyther way. But I can see how compatabilaty is disierd especily with a mood like this so... Submod? or maybe a very clever decision that cann be ignord if it leads to problems?

Creating a new goverment for mages sounds very immersive.

What are we even gonna get from making a mage tower its own holding?

I was thinking about adding multiple buildings like libraries, Arcana batteries. etc. Adding more complexity to the mage's tower and more customization options. I even thought about a whole mechanic to add something like electricity that can be used to unkeep building and you must build something like power plants. everything with theme of arcana

I think the biggest priority is to add more spells to arcane legacy and abilities.

Can't it be done just by buildings requiring other buildings in the holding? I don't know how ck3 handles buildings.

I'm not sure you are ready no wreck compatibility right now. Maybe focus on things that can work with other mods, so there are no hurdles for people installing AL2. Then, when its feature rich and popular, you can afford losing a bit of people by making it its own thing.

I can do this by decision. Because to add buildable building you must replace holding with your own.
By the way. I have now big problem. I don't know what to add next...

Take a page from Elf Destiny's book and add expeditions? Have different diffictulies, if possible. Add challenges, and at the end - some reward: unique and common items, arcana replenishment potions, rare arcana enchancement potions, unique spells that you can't get from the lifestyle etc.
This one, I think, should be doable.

Much more ambitious and maybe not feasible goal would be to make AI use magic. Maybe not all of it, but some enchancement, healing etc.

Great Idea!

when going inside your tower maybe add decisions were you can build and expand your library, laboratories, workshop, garden, ect. then later you can build arcane artifacts, maybe summon/bind spirits turning them into highly skilled courtiers, unlock clockwork men-at-arms, forging then later upgrading your mage's staff and robes, making rings and gifting them to your follow rulers while you keep one in secret...
more events while in the tower would be super cool, like picking and train apprentices, spouses trying to drag you out of the tower and spend time with your mother in law :D, a mysteries knight knocking at your door asking for advice on how to slay... and so on

I must tell you that your's idea are the best. In 1.3 i will think about adding most of it!