I struggle to post because I'm a perfectionist. I'm hopping onto Dyson's DICEMBER2021 challenge to try and break that (I'm only 1 day late, I might go back and make up the ammunition prompt this weekend). Here we go with the ICE prompt!
There are a few rules in D&D that are as evocative as they are difficult to implement. One that always gets my imagination spinning is Magical Research:
"It is also possible for a magic-user, through expenditure of much money for research, to create new spells. The Dungeon Master must agree that the spell is appropriate to the level of the character trying to create it, the magic-user must spend 2000 gold pieces per level of the spell and one week of time. He then has a 20% chance of success." -Holmes Basic D&D
Magic always felt like it walked a line between science and art; on one hand governed by incomprehensible cosmic laws and on the other a deeply personal expression of the wielder. Here is an exploration into the creation of an Ice Mage (although this system could be expanded to all kinds of magical disciplines).
Here we go, a completely untested mechanic to destroy your game!
At level 1 a Magic User may choose a Discipline (examples: Ice, Fire, Storm, Undeath, etc). The default is a generalist which treats all spells as non-discipline.
When finding a spell scroll or spell book the caster may make a check to add the spell to their spell book. Each spell may only be checked once per level to see if it is learned. Attempting to learn a spell from a scroll uses of the scroll whether or not the check is successful.
Before attempting to learn a found spell the Dungeon Master determines if the spell is within the Magic User's discipline, outside of their discipline, or antithetical to their discipline. Magic Users have a greater chance to learn spells within their discipline and a lesser chance to learn spells antithetical to their discipline.
A magic user may spend 2000 gold pieces per spell level and one week of time to re-interpret or convert a found spell to their chosen discipline before attempting to learn it (thus increasing their chances of ultimately adding the spell to their spell book). Generalist magic users may not do this.
I would take this further, only allowing magic users to prepare one copy of a given spell. However, one could imagine a magic user preparing Icicle Volley (magic missile), Spark Shower (magic missile), and Fire Arrow (magic missile).
Some example "converted" spells:
Icicle Volley (Magic Missile): Auto-hit, 1d6+1 piercing, +1 missile/5 levels, x2 vs. fire creature (the piercing damage reduces the effectiveness vs. undead offsetting the bonus vs. fire).
Glacial Armor (Shield): AC2 vs. Missiles, AC4 vs. Melee. Successful melee hit shatters armor ending effect but dealing 1d6 damage to attacker.
Freeze Shut (Hold Portal): Open with fire based spell/ability or forced open by creature with +3HD more than the caster.
Cold Snap (Web): Frozen creatures can't move (Str to break free), flames negate effects
Freeze Solid (Hold Person): Paralyze (save vs. spells)
Bitter Cold (Cloudkill): Freezing fog moves 20'/round dealing 1 hp damage/round to all creatures in contact with it. <5 HD also save vs. death.
Flash Freeze (Transmute Rock to Mud): Transmute 3,000 square feet of water into 10' sheet of ice. Creatures caught in/walking on ice move 10% regular speed.