If this is added into the game as a DLC, maybe call it "Brave New Spell System", I would buy it in a heartbeat. I'd also enthusiastically suggest a "divine spell" category be added which will give a nice boost. So if anyone has enough mana they can cast.
1 a Bane / attack spell (fireball, weakness, firestorm ...)
2 a Heal / Blessing (heal, frost weapon, elemental resistance ... )
3 a summon spell (ghost wolves, earth elemental...)
4 a spell in the Others category (counterspell, teleportation, greater dispel ...)
5 a divine spell (which can be any type really. This adds a nice incentive to go with a God to augment your playspell. e.g. go with Aegrelia = two heal spells per turn (if you have enough mana).)
The brilliant thing about the system you've proposed is that you'd have to think about how you're going to spend your mana. "Do I spend a bit more on a stronger fireball now and leave less mana to summon a support unit, or take a risk and wait so that I can cast that powerful divine spell?" That just sounds like how the game should feel... but it doesn't. I just pick spells at random really... and then wait... and finally cast and meh, pick whatever next one.
It would also be great if the current randomized spell research tree could also be changed to reflect these 5 categories. Let's say a player can always choose to research a spell from one of the categories which leads to more spells being available for research *in that category.* In other words: spell development tree. AI can then be customized to make either a "attack heavy," "summon heavy," "balanced," "heal heavy" etc. type of mage.
I don't suppose DLC's from released games can try to get Steam Greenlit?
FYI, Warlock 2 is announced. I would not expect any drastic changes to the predecessor now
Feel free to post feedback what you would like to see in Warlock 2 in the appropriate forums: http://forum.paradoxplaza.com/forum/forumdisplay.php?834-Warlock-2-The-Exiled