Bounds execption: Allows larger objects to fit in smaller containers, ritual spell effective for 1 day, can be upcast to increase amount of "overfill", some check when retrieving the item for a chance of "corruption" (small chance to retrieve a different item, or have the "overfill" become something different like have the outer edge of a bar of gold be meat)
Paralellization: High level time magic, Allows the target multiple turns to be taken simultaneously, but an equivalent cooldown afterwards (waiting for all threads or jobs to return before being able to continue), so 3 turns at once means 2 turns the target can't do anything after.
Split brain configuration: Allows focusing on multiple concentration checks/spells at once. This could also be called paralellization, or multithreaded.
Pass By Reference enchantment: requires two identical items. Once enchanted, changes to one happen to both. Room for all sorts of shenanigans.
Private field: cast on an area to prevent entry/visibility into it by unauthorized entities. Sounds inside are not audible to unauthorized entities outside of it.
Parallelization would make high level casters utterly broken. One of my favorite spells, because I made it for 3.5 edition, and I haven't had a DM criticize the spell, is Unfailing Missiles.
If I could cast three of them in parallel, I would be able to hit up to 9 targets that are no more than 20° arcs from the last target, each with a missile that does 17d (6+1) points of Force/Sonic Damage split 50/50. Or I can use all of those missile on a single target. If I maximize the spell with 3 12th level spell slots, that's 1,071 damage or 119 damage per missile. There's no save, please refer to the name of the spell. This is basically a 9th level Magic Missile. If I see you, I wollop you.
I would then need to hide for 12 seconds. Not particularly difficult if one has a Contingencied Improved Invisibility, or a Contingencied Screen.
Split brain configuration: Allows focusing on multiple concentration checks/spells at once. This could also be called paralellization, or multithreaded.
Allows you to concentrate on two concentration spells instead of just one. However, the spell itself is concentration.
I would abuse parallelization. Moon druid in owlbear shape with three hits for each action (through druid features and magic items) and a free action each kill (from an elixir), doing three of those at once would end most fights