What would happen if spells took a round to resolve?

Question

I am considering the potential implications of a house rule for Dungeons and Dragons 5th Edition. The rule is as follows:

1. When a creature casts a spell of level 1 or higher with a casting time of 1 action or 1 bonus action, they follow the existing procedure for casting a spell, including choosing locations and targets, providing components, and so on. However, the spell does not take effect immediately. Instead, the spell's effects occur (for spells with instant duration) or begin (for spells with non-instant duration) at the beginning of that character's next turn. The spell slot and/or consumable components, if applicable, are consumed if and when the spell takes effect.
2. If a creature takes damage after they begin casting a spell but before it takes effect, they must make a concentration check, using the existing rules. They must make this check for each spell they are casting. If the check is failed, the caster stops casting the spell and it will not take effect at the beginning of their next turn. The caster may also choose to stop casting the spell at any time before the beginning of their next turn.
3. If a chosen target of the spell is no longer a valid target when the spell takes effect, the spell takes effect but does not affect that target.

How does this house rule interact with the existing rules as written? Does it produce any ambiguities or counterintuitive results?

Answer 1

This would make spellcasting in combat too weak

First off, it technically may not help as much as you wish with targeting, because consensus is that spell targets are only selected when the spell resolves. But let's assume that you mean if a target moves between the original cast and resolution, this can affect targeting: then this would make most to-hit attack spells a waste of time if the targets can move. In a world where magic works like this, you would expect creatures to constantly move to avoid spell effects.

Secondly, most combats only take about 3 rounds. Even if there was no change to how effective the spells now are, this would effectively waste one of their 3 turns for the spellcaster, and make them about 30% weaker.

This will have a negative effect on the overall combat balance of the game, because many parties have about half of their team as full casters, and those now can't carry their weight in combat. Also note that this will also weaken those characters who only cast spells to supplement their mundane combat abilities, like a ranger: their hunter's mark now only works for 2/3s of the fight. Because many monsters do not use any spellcasting, this is a major imbalancing of the game against the players.

Frankly, with combat being a major part of the game, you can just expect that no-one will sign up to play a full caster under these rules. The classes are pretty well balanced in combat, and majorly nerfing only some of them will make then non-viable vs the others, at least for tables where balance matters.

It also does little to help with your stated goal of informing creature‘s actions against spellcasters, other than moving around a lot and preferably attacking casters if they can to disrupt their spells: without an Intelligence (Arcana) check, you would not know what spell is being cast, nevermind where it was targeted. And it's not as if even without this rule combatants tend to try and maximize the effect of their spells and protect their squishy casters, because concentration is already a thing.

Lastly, this will do little to curb the reality-warping out-of-combat powers of spellcasters that often are what makes them feel better than martial characters, because out of combat, a round matters little. They can still scry, can still teleport, can still cast wish to create 50,000 gp diamonds, can still resurrect the dead, can still summon angels to to their bidding.

This is a terrible house rule, and will not achieve your objectives.