Apologies if I post some that are already suggested. I'd read through the others but I actually wouldn't and didn't.
1.
A weaker kind of breach that can be given to spoopy classes and paths/domains that specialise in necromancy, including undying warlocks, undeath/death domain clerics, defilers, wizard specialists, etc all the stuff.
The breach would be in the necromancy school, and it would only affect/strip other spells related to necromancy, such as deathward, nep and shadow shield. That kinda stuff. More generic spells like SR, spell mantle, etc would still be unbreachable by this spell.
2.
Mass baleful polymorph (like the wildmage surge)
3.
Might be better for ESF: Trans than a spell anyone can use, but something like an ability to set an anchor point where you currently are, and be able to teleport back to it. It could consume your ESF: trans ability for the day, and it could cost GSF ability to teleport yourself or someone else to that point. Kind of like a reverse yoink. Yes/no prompt so it can't be weaponised. All that stuff.
4.
Some kind of Shadow-Door/Shadow-Step ability that Shadow Mages and Shadow Dancers can use to move around the screen, that isn't tied to the -guard shadow. God I hate that shadow.
5.
Mage Hand. It would just let you interact with objects from afar. Open a door. Pull a lever. Much like gust of wind, but without the woosh. Not really used for much except niche situations and for RP.
6.
A very high level, costly (component/piety depending on version) spell, like wish (divine intervention for cleric), that just makes an impressive visual or has a small selection of impressive visuals. It wouldn't actually do anything, that would be left up to the players to RP, and it would just serve as a visual aide to that RP.
7.
Change weather for druids. Although realistically druids would be unwilling to mess with the delicate balance of nature and an ecosystem by just changing the weather.. This is d&d and a video game, so it would just be cool.
8.
Spells that turn you into objects rather than creatures. You would be unable to move (except maybe super super super slowly). Become a chest. A chair. A barrel. A tree. A shrub. Give it to druids so druids can be shrubs. Reject modernity, return to shrub.
9.
Magnificent Mansion- A way I saw it done on a server I used to DM way back in ye olde days. It would spawn a tent/something, and when interacted with you go inside and it's a big quarter. Something like that. Gives players a chance to go inside and rest with their party no matter where they are. Doesn't make you immune from PvP if you destroy the tent, because someone else could access the same pocket dimension by casting their own. I dunno.
10.
Maze, but really. You stay there until you find your way out. You can only maze 1 person at a time, so you can't just get rid of all your enemies by mazing them. Current maze sucks, you fish for 1s, and then if you fail you're just forced to watch 2 loading screens and things continue as normal.
11.
Animation spells, but they create undead that are NOT under your control and do not count towards your minion cap. They're not hostile to the caster. They just stand/roam about waiting for victims to aggro onto. This would let necromancers spook up an area and leave. They wouldn't be strong or anything, just very offensive (morally, and pungently).
12.
Dimension door. Same as Shadow Door/Shadow Step. Same same, but different. Maybe it has longer range. Or shorter. Maybe it works during the day while the other only works at night/not day areas.
13.
Greater Fireball/Lightning Bolt. It's like Fireball/Lightning Bolt, but it's Greater. Higher circle, much more damage. Classic spells I wish had more use but they're so weak and easily saved vs. Maybe the greater versions offer only half damage on success or something. The visuals are awesome in NWN. I love these spells, ESPECIALLY FIREBALL. It's like, the coolest looking spell in the game and a real pity no one uses it.
Stopping at 13 to curse forums with bad luck.