I have been thinking about the Tower Tournaments that MetaZoo is putting on this year (and presumably the next, as Mike has said there will be tournaments for the missing aura types), and I have a theory on the spellbook restrictions.
(If you are not in the know; MetaZoo is hosting some huge tournaments this year, and each tournament has a specific aura type theme, that your deck has to match by being at least 60% that aura type.)
First off - if you've been in the MetaZoo discord, you've no doubt seen the complaining about the 60% aura restriction. A LOT of these complaints are either about a) how few cards are left over after you've added your aura cards and met the minimum and/or b) how little is left after you add your 'staples' - Power-Up Red, New Beginnings, Second Anniversary, etc, etc.
Secondly - a lot of people are focusing REALLY hard on the 60% minium being from a 40-card deck. A lot. Which is weird, because...
Thirdly - the first Aura Type to be showcased in these Tower Tournaments is water.
Now, here's the thing; water is a very particular aura type. Hands-down, across the game, it has one defining common feature - it is a bookmarking aura. It has incredible draw power. Tons and tons of Water cards let you draw extra cards - more than any other type of Aura in the game, maybe even all of them combined. That means you can play larger decks, but they still play fast, which is the defining problem in tournament decks right now - everything has to play fast, because you're fighting against the top 3-5 tournament winning decks that everyone plays, and most of them are fast.
Another thing worth mentioning in this; Mike, the creator, has stated a few times that he wants to stay away from banning cards, and there's no plans to have sets move out of rotation. The intention is the whole game stays playable indefinitely. This is all fine and well, but it does potentially raise a problem because right now...
... Most of the tournament winning decks are all the same. Mostly lightning. Some cosmic. A lot of flame. The top 3-5 decks to beat are always the same. They all have those 'staples' - Power-Up Red, New Beginnings, Second Anniversary, etc. They are all 40-card decks, or very close. Anyone who wants to build a new 'good' deck usually builds one of the main ones, because guess what - they win. Which brings me to my theory.
MetaZoo is trying to change that. They see a tournament scene dominated by very small, identical decks, with incredibly similar cards in every tournament. They want to change that, to have people explore combos that haven't really touched the scene as much and actually sit down and build new decks, instead of copying the master list of the main decks. And they can't get you to do that without banning cards or decks, so they're doing the next best thing; they're making you build decks around specific aura types.
Honestly? It's brilliant. And the fact that they started with water is an interesting choice, because water has incredibly draw power. It allows you to build bigger decks without sacrificing the speed you're used to, which means, in theory - deck sizes should increase. Not only are they forcing people to move away from The Staples, they are also giving them the chance to expand the standard deck size without it feeling forced or unusual - it's a natural step forward, which means the next Tower will probably see a rise in deck sizes as well, which means a continued diversity in strategy all over again.
(I'd like to take a moment to thank @samsquanch for this one, actually - I had already had my theory, but his topic on water bookmarking added a whole other layer to it.)
I'll also add that it feels like they're particularly focusing on aura types this year that are either underplayed, or usually win by default. Water hasn't really topped any of Caster Society's charts, but the next type will be Earth, which is very underplayed, and the last two are Dark and Spirit, which are popular, but get knocked out of the running most of the time. The two middle aura types are also interesting; Lightning and Flame. Two incredibly popular aura types, that almost always win - but how will that play out in a room full of a 150~ people ALL playing the exact same decks? People will have to get clever, if you ask me. And I think that's exactly what they want.
Kinda defeats the purpose of you ask me.