> If you have another idea that also fulfills the needs while actually being more newbie friendly, feel free to open a discussion about it - nothing is carved in stone at this point.

Without reading the Rest of this thread, here it is:
Just a simple taught, a great difference between OpenClonk, and all the old Clonks is the usage of the mouse. So why don't use, kind of mousegestures to cast spells. As a player you have a staff, like a normal weapon in your hand. The staff only contains a few spells, that are all kind of related with each other, for example three ice-spells. While playing you press the mouse, and move it in radial direction. This will cause, for example the frostbolt (ger: frostbolzen), when you move your mouse in circumference direction it will cause an other spell, maybe a iceshield, that vanishes after a short time. So you can cast your spells fast and intuitive. An important aspect is that there is a length of the way of your mouse, this could be an input for the strength of the spell and how many mana it will use. The distance to your clonk can be used as well, so to evoke the shield it will use up more mana the more far it is away. Alternatively you can use the x and y axis - maybe for other kinds of spells.
Additive to moving your mouse you can just click on a point. This could be used for more tool-like spells, for example heating which melts ice to water, stone to lava, sets trees on fire, and hurts clonks, or freezing water to ice, lava to stone and freezing clonks for a moment. Here you can't use the way to power up your spell, but the time. So you aim on a rock, click and wait (the stronger you are, the faster it works), first you just melt a little area, like a pixel, then 5 pixels, then more, and more, so that you get an expanding circle around your pointer. The bigger the circle is, the more time it takes to expand, and the more mana it will use. Here the distance to your clonk is important for the power and usage of mana too.
This way you get like three spells in one staff, and with two hand, you can only manage six spells at once. So, when you want to have it more complex you can make more complex gestures. You can draw Z and then make a + where your target is, and a flash will hit it. That would allow combos too. But that's kind of complex for programming.
So only think of six spells, you can still combine them, in another way. As an example: You use a shockwave (special-jedi-forces-like-staff - circle) to throw your enemy in water and than freeze it (ice-staff - click). Or you melt rock to a piece of lava and then hunt your enemy with telekinesis-flying-lava (special-jedi-forces-like-staff - click, to grab the lava, than move while holding).

This concept is fast in use and easy, but it limits the amount of spells by the number of staffs (and other stuff) you can carry.

edit: a feature (not necessary for this system): when you have used up all your mana, it will use up your health. This would make it kind of more dramatical.
Kind of: Moses standing on the sea, lifts the water up, his folks walk thru the sea. With the last power of his health the last child passes by, then - nearly dying - he lets the water smash the egyptian army. (Please, don't take me serious.)