Hey, Kalavinka!
Here are some pointers from our design team
The principles that you identified are indeed some of what we use internally
With the exception of "Each skill must be unique" - we strive for that, but if gameplay on the whole would benefit from two very similar skills existing in different schools, we'd go for it.
Also note that there is a very big difference between adding skills to existing schools and adding a completely new school - a new school brings many unique problems to solve. Principles below concern the former case.
For existing schools, an important consideration is at which point the skill becomes available in the game.
Our starting skills (first half of Act1) are made to be easy to understand and plan around, but also covering enough design space to serve as the cornerstone of a character's combat kit.
Skills that appear in the second half of Act1 usually deal with magic and physical armour, as the player needs to be comfortable with managing those by the time they leave Act1.
For Act1 a good trick is to make a skill that is straightforward mechanically but give it some fun flavour to make it attractive.
Act2 is mainly about mobility skills and exploring every bit of design space we have. There are fights in Act2 that will challenge the player to master surfaces, resistances, mind control, mobility, target priority, etc. A good skill for Act2 is something specialized that can solve narrow problems but also combo well with other skills.
In the endgame we tried to make fights that will challenge the player to try tactics outside of their comfort zone. This is a good place to put a skill that has a significant drawback that needs to be played around (e.g. piercing self-damage, or disabling the caster for a turn), but will be used by expert players for greater advantage. It is also time for skills that serve as a combo-topper, something that rewards the players for prepping the battlefield and manipulating enemy positioning with a fireworks display.
Finally we tried to include skills throughout the game that, when put together, will lead to near-degenerate tactics in the hands of the most dedicated players. Typically these are skills granting extra AP, SP, turns, resetting cooldowns. It would be a good idea to include in your batch of skills something that will ignite the imagination of players that want to break the game in a way worthy of a youtube video.
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