Balance in HotA.

There were many ideas surrounding a reasonable redesign. I don't recall seeing this specific one, and it doesn't seem to be the most reasonable of the proposed options, but let's keep it in mind just in case.

This option has probably already been suggested, but I'll write it down just in case: when you click on a spell in the guild, it offers to replace it with another (random) spell for a certain price.

In my opinion, after adding portal gates to the map, from which you can teleport to any Castle without an Earth Expert (and without Wisdom), control spells naturally become universal.

No, adding Teleport and Dimension Door to all schools would definitely be nonsense. You just need to create another object on the map for Dimension Door and for new control spells, if they are added. Also, just in case, I'll remind you about a very cool object that was shown in the very first Photoshop screenshots of Heroes of Might and Magic, - a broken portal that moves the hero to a random point on the map.