Lengthened Spellcasting (3.5e Variant Rule)

Lengthened Spellcasting
This variant attempts to fix a number of things inherent within the 3.5 system;
 * 1) Many casters are often overpowered due to spells that can potentially cripple the enemy without any chance of retaliation on a single failed save, or spells that don't even give a save at all. This variant gives enemies an opportunity to interrupt spellcasting.
 * 2) Majestic spells of awesome power with long incantations don't seem to fit taking the fraction of a round that characters get. (Since a whole round is approximately 6 seconds, and a character gets only a small part of that for their round.) This variant allows a character to incorporate words of arcane power, rituals such as lighting a candle, or intense hand gestures into their spells where before one barely had time to say more than two words.
 * 3) One usually has to ready a Dispel Magic to counterspell an opponent's spell, which sometimes leads to rounds wasted especially if one doesn't succeed in counterspelling.

To fix these issues, we have the following variant:

The casting times of spells (and other spell-like effects) are lengthened to one round, casting beginning at the start of the caster's turn and ending at the beginning of their next turn. Spells that normally take a swift action to cast (including Quickened spells) now take a standard action to cast. Immediate action spells may still be cast out of turn, but "use up" the caster's standard action on the following round. The caster may designate their target either when they begin casting (good for touch spells, for example) or when they complete the spell (better in the case of area-affecting spells). Alternatively, (in case all targets move out of range for example,) they may instead choose to cancel the casting of the spell at the last moment as a free action and are treated as though they hadn't tried to cast it for the purposes of spells per day.

There are two primary exceptions to this rule, spells from the Conjuration (Healing) school (or other spells whose main purpose is to heal or remove negative status effects), and spells with an Instantaneous duration whose main purpose is damage. (The second is primarily decided by the DM, but a good guideline is a spell that deals damage is one whose dice of damage at the minimum caster level equal or exceed one's caster level. Examples include Scorching Ray, Fireball, Cone of Cold, and the like.)

A third exception to the rule is Dispel Magic (and its Greater version) when used in the counterspelling capacity. If used thus, casting completes when the target's spell's casting is complete.

This also applies to the use of magical items that reproduce spells and the like, allowing enemies to attempt to destroy the object before the activation is complete.

The above rules also affect spell-like abilities. Supernatural abilities, martial maneuvers, and the like, on the other hand, work as normal.

Adaptation
A more limited version of this variant, taken with inspiration from this variant, is to have it only affect spells of a certain level.

Have the above rules in place, except that casters ignore them as long as the spell level is equal or less than one-fourth their effective caster level, rounded up. Thus, a 10th level caster would be able to normally cast spells of up to 3th level, but 4th and 5th-level spells would require a 1-round action to cast. Alternatively, have only the two highest levels of spells that the caster could cast be affected by this variant.