How would changing the damage type/saving throw from spells affect game balance?

Question

So, let's say we want a mage character based on ice, sure, there are quite a few spells with Cold damage/thematic, but they're not quite what you want... You want him to have a Fireball that deals Cold damage, you want a Frostball, Iceblast, Frost Explosion, call it whatever you want!

So, if I took spells like Fireball, and changed the damage type to ice and the saving throw to CON, would that affect the game balance?

Another example, let's take Sunbeam and turn it into... Darkbeam? It now deals necrotic damage instead of radiant.

I'm aware of this question: What are the most and least-resisted damage types?, I know that a Fireball that deals Psychic Damage and requires an Int save is stronger.

So let's classify saves in two categories: The "common", Dex, Con and Wis, and the "exotic", STR, INT, CHA.

So, if I swap "similar" damage types (fire/cold, necrotic/radiant, etc) and also swap saving throws of the same "category" (Dex => Con/Con => Wis, etc) would that mess up game balance?

Answer 1

WoTC have released an Unearthed Arcana Wizard subclass called Lore Master. One of the features this subclass gives is:

Spell Secrets

At 2nd level, you master the first in a series of arcane secrets uncovered by your extensive studies.

When you cast a spell with a spell slot and the spell deals acid, cold, fire, force, lightning, necrotic, radiant, or thunder damage, you can substitute that damage type with one other type from that list (you can change only one damage type per casting of a spell). You replace one energy type for another by altering the spell’s formula as you cast it.

When you cast a spell with a spell slot and the spell requires a saving throw, you can change the saving throw from one ability score to another of your choice. Once you change a saving throw in this way, you can’t do so again until you finish a short or long rest.

This seems to cover both effects that you wish your Mage type character to have.

So WoTC appear to be looking at the same sort of concept that you want for your homebrew campaign.

Looking at this trait however it is evident that WoTC, at least, consider the changing of damage type to be not that big of a deal to game balance, but the changing of the type of saving throw to have a potentially greater effect (and thus needed to be restricted).

Taking your fireball example, changing the saving throw from DEX to anything else would eliminate the rogue’s evasion class feature.

In general the saving throws have been sculpted to the type of effect the spell is imposing.

Some salient examples are:

• area of effect spells are largely DEX saves (fireball)
• a spell effecting a targets mental faculties are usually WIS or INT saves (Fear or Feeblemind)

Important Caveat

It obviously comes with the caveat that this subclass hasn’t been playtested as extensively as the subclasses published in the books, and as a result is not necessarily balanced within the game.

Answer 2

Swapping Saves: Yes. Swapping Damage Types: It depends.

Saving Throw types are a fundamental "architectural" element of the game, on a level with characteristics and classes, and tampering with them can have a profound ripple effect throughout the game. For example, changing a save from Dex to Con means doesn't just mean that the spell is somewhat less effective against burly barbarians and more effective against slippery rogues. It means that a Rogue can't use his Evasion class feature - one of the signature features of the class. As a DM, you can do this occasionally as a way to buff up an encounter without much affecting the overall balance, but to allow players to build characters around the idea invites abuse.

Damage Types, however, are not integral to the game itself, but to the setting. Resistance to Fire, for example, is particularly common in the Monster Manual, but in any particular campaign it might be uncommon or even absent. A campaign set on the Ice Planet Hoth might have no creatures with resistance to fire (or, perhaps, every creature has resistance to fire). As long as the players know this going in, they can choose spells, feats and features that reflect the campaign reality.

Thus it is entirely possible for the DM to allow variant spells that change the damage type without affecting play balance. All that is required is a modicum of attention to the specifics. In an undead-heavy campaign, changing Fireball to Radiant damage would be a huge change, but in a wilderness campaign it is probably fairly trivial (depending on what is populating that wilderness).

Of course, this is true only if the spell list is under the control of the DM and the player can't change the damage types on an game-by-game or adventure-by-adventure basis. To be able to tailor the damage type to a specific encounter is of huge benefit, as it renders resistances all but moot and it amplifies vulnerabilities tremendously.