What impact does this "Spell Melding" homebrew feature have on the balance of casters?

Question

I am currently homebrewing alternative types of casters for D&D 5e, and mostly looking at making them unique through new ways of casting spells. My main objective is to keep them in line with the current casters and their strong points, without making the original ones irrelevant or underpowered in comparison (this is what I define as "balanced" in the context of this question).

I've refined one of those ways of casting, and am now under the process of balancing it. However, I am aware that spells and spellcasting in general can easily be abused or broken, and thus I am looking for feedback on the following ability.

For the context of the ability itself, it will either be the core feature of a new wizard-like full caster, or an ability directly granted to a wizard, either as a feature or a magic item. As such, you may evaluate this with a wizard's spell list, although if there are interesting things to point out with spells not in this spell list, I'd be glad to hear about them, in case I ever allow multiclassing.

Here is my current take on the ability which I've currently named Spell Melding (still haven't found a name I'm satisfied with, but that can wait) :

Spell Melding. Experimentation around spell-storing magic has given you the ability to hold a spell's harming potential, and inject it back into another spell later on.

When you cast a spell that inflicts damage, you can instead choose to store the spell's damaging aspect. You can only store one damaging aspect in this way, and storing another damaging aspect erases the currently stored aspect. Any component and spell slot used during the casting is consumed as usual, and the spell is considered as cast successfully.

When storing a damaging aspect, roll the damage of the spell as if you cast it normally. The damaging aspect retains the spell's final damage values after applying modifiers (outside of target-related modifiers such as weakness and resistance) and its damage types.

When you deal damage with a spell, you may inject your currently stored damaging aspect into the damage-dealing spell. The damage of the stored aspect is added to the current spell's total damage, and retains its original damage types. The stored aspect is consumed in the process.

A stored aspect cannot be used on the same round it was stored. Additionally, a stored aspect vanishes after being stored for 1 minute.

The description is quite long, as I tried to limit any unintended interaction that could break everything. The basic idea behind this feature is to give the ability to "hold" onto the damage of a spell, then release it alongside another spell later in the combat, allowing to deal large amounts of damage in a single attack.

The main goal of this ability is to give an alternative, more risky way to use spells, but with much larger bursts of damage. It makes you use your action on a turn to technically do "nothing", but allows a much larger damage output on your next turn, which depends on the resources you've spent during your previous turn.

Here are the main concerns I currently have about this feature :

• Combat can be fairly short, no more than a few rounds, so this feature may be too slow to be fully taken advantage of.
• There may be ways to abuse how this feature is written in order to deal damage much higher than intended.

In the situation where this feature would be slightly unbalanced, but not to the point of having to totally reworking it, I have the following solutions :

• If too weak, instead of entirely consuming the stored damaging aspect, only half of it could be consumed, while keeping the full damage bonus applied. This would result in getting 1.5 times your original stored aspect's value, over the course of 3 spell castings.
• If too strong, injecting the stored aspect into a spell may require a bonus action, instead of being free in the action economy.

Is this ability balanced for a wizard-like full caster, when compared to the current full casters?

Answer 1

As per your comment, I'm going to consider only single target spell damage stored, and applying it to single target only too.

It skips saves and defenses

If you have a high damage spell, you just store the damage before battle. Then, on the first round, you cast a magic missile. No save, and you have three targets so there's a high probability that one or two will not cast Shield. And now you can apply your high damage from your highest level spell without giving the target opportunity for a save, or benefit of Armor Class.

Even if you won't combine it with Magic Missile, you only decide to apply the damage when the spell you are using actually deals the damage, so only after target already failed their saving throw. And you apply it in full even if the spell was about to apply only half of the damage, because it deals half on successful save.

It doubles your first round effectiveness

You have the damage output of two actions at the cost of one. At least, because there are 4 damaging spells with casting time longer than 1 action. For this alone, it would be on par with other abilities that allows classes to get extra action, but probably more powerful than what, for example, Fighter can do with his Action Surge. And Fighter only has it 1/long rest until 17th level. Your feature has no such limit.

It effectively removes one of the spellcasting limits

PHB p.203:

Bonus Action A spell cast with a bonus action is especially swift. You must use a bonus action on your turn to cast the spell, provided that you haven’t already taken a bonus action this turn. You can’t cast another spell during the same turn, except for a cantrip with a casting time of 1 action.

With your feature, you can cause damage of two spells using a bonus action (there are a lot of spells to do that) and still have your main action available for anything you want, for example a cantrip. Nothing, even Sorcerer's Quickened Spell, allows you to cram that many spells in one turn, so the assumption that it was intentional seems warranted.

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I would probably not allow it at my table as is for now. For every one of these issues, I would be open to playtesting it, but all three seem a bit much to me.

Answer 2

It has some weird interactions with area of effects

Mainly the same effect either way; first cast something such as "Meteor swarm" at your party before combat starts, what would the effect of storing this be, does it store one hit or all the damage? All would one shot almost anything, but it isn't unreasonable to read it like that currently. The same question is also in reverse, what happens if I use this charge on a spell like upcasted Magic Missiles to hit 8 enemies with a damaging spell meant for a single target?

Along with this, on the topic of Magic Missiles or spells that only damage enemies such as Wither and Bloom, it can be used as a discount version of the evocation wizards spell sculpting of; use big spell that would wipe out your melee party > put that damage onto a spell that only hits enemies.