You might try this:
Thank you Chthon. I feel we need to create a new topic, where we can write all unrealized interesting concepts and solutions for this. The last 4 years I have seen a lot of interesting.

Well, I was quite satisfied with the Spell system, but I really want to realize Throwing items - grenades/bombs/Molotov cocktails. I feel that your idea can help me in this.
Ha, so your "passives" are somehow equal to my attributes i guess and the player has some more possibilities then Foc,Dex,Str to choose from.
Well, my "passives" have a long story, I reworked it several times

, and now it works like the standard passives with tier-bonuses.
But before I start there is one last question: when the spell damage output is based on your Stats, why all the other requirements? Fireball on a mage will deal lots of damage while Warriors might use it as an ordinary spell and the other way around.
Well, the
Weapon requirement is necessary for technical reasons- Spells for all Weapon types need too many animations. For universal spells is needed a universal animation, for Special spells - special animation. It is much easier to ban spellcasting with an inappropriate weapons, than doing animation for unused spells.
Example:
Arc BeamIn theory, you can emit a beam using a sword/hammer/spear maybe even a bow.
Perhaps this is a bit illogical, but it can be implemented. We just need Arc Beam animation for all weapons. But do we really need this?
Let's say that Weapon Damage Spells - is weapon-logical spells.
Exploding Arrow - for bows.
Seismic Slam - for hammers.
Cleave - for axes and swords.
Frosty Shield - for staves and wands
etc.
Meanwhile your question gave me food for thought...
My plan - to use the Weapon Damage for spells instead of WDPS, but what about the one-handed weapon? In this case, one-handed weapons always lose. 
Maybe cast animations for one-handed weapons should be faster? This could solve the problem, but it does not solve the mana cost problem(you have the same cost, but inflicts different damage).
I need to think about it.
Mana Cost requirementWell, it is rather a stimulus to maximize Mana. A player must decide what is more important - Mana or other bonuses. If you have the same mana cost for all similar spells, then Mana just an unnecessary secondary bonus/stat.
For Warrior / Rangers we have low mana cost, Mana is a secondary resource/bonus. You can collect other more useful bonuses - speed, health, armor.
For Mages - big mana cost, Mana is a main resource/bonus. At first - collect Mana and then think about other bonuses.
In addition, different mana cost shows the difference between the "warriors" and "mages". Ie you're cool Mage and you have a lot of mana - you can use powerful spells or you are weak Mage and you have enough mana - you can`t use powerful spells.

Mana Cost teaches how to be a Mage - сoncentration, sacrifice and prudence.
Also Exclusive "class" spells - it's an incentive to try different classes.
For spells spread between all classes there might be a 4th category.
Or more simple, a color-system similar to item rarity so you just need to frame the icons.
I'm not sure if that is what you meant by "planing" or if it's more about the spells categorization and graduation..
Yeah, exactly - the color indication. Now it is only a concept, but I used the standard colors:

Red - melee weapon spells
Green - range weapon spells
Blue - magic weapon spells
Purple - universal spells