I'm just referencing the spells off the Roll20 site (e.g.,
List of Druid Spells by Level)
I'm mostly working on spells that I'm almost positive won't be added by Larian. But I'd say here's the list of spells that are mechanically fairly easy, interesting, and don't already have an obvious corollary. Other people could add spells that work largely as existing ones, but these are the ones I like. Listing from least to most powerful, the latter of which would probably be available at level 18+ in a GM campaign and would probably have major requirements (source points, special inventory items, etc.)
Faerie Fire: Mass Dodge Malus
Healing Word: 0 AP small heal with long CD maybe
Jump: grant a character a farther distance leap skill
Animal Friendship/Dominate Beast: a charm that can only target animals
Heat Metal: deal damage based on armor of character
Pass Without Trace: Mass +Sneaking buff
Conjure Animals: might be able to make a random summon spell. Same with conjuring elementals.
Glyph of Warding: Explosive, magic based trap, should be possible
Freedom of Movement: Movement speed buff, immunity to slow, cripple, stagger, any other movement debuffs
Contagion: An earth-based disease spell
Antilife Shield: Each turn blast out a force that pushes characters away?
Teleportation Circle: A massive teleport skill would be fun, though very difficult to script in a meaningful way probably. Something like you can teleport you and allies within 8m up to 15m away, retaining the same formation. Not sure if the AI grid will automatically shunt characters into walkable spaces or what, but probably not.
Dream: This a fun concept. Would tweak it such that it could be a combat ability that would make you and a target asleep, and when that target wakes up, they take moderate damage. Maybe if cast on an ally, you'd both fall asleep and they would heal when waking up.
Mislead: Teleport a distance, go invisible and summon something that either looks like you (if possible), or some other decoy kind of summon. A nice variant for a summoner skill could be a short range teleport where you plop down a totem where you just were.
Eyebite: Such a silly spell name lol. But like the concept of a spell that when you and an enemy look directly at each other, they receive a random negative debuff.
Sunbeam: Line of fire-damage that blinds
Feeblemind: Permanently or otherwise stupefy a target, reducing intelligence and charisma by a lot and muting them.
Foresight: Large Hit Chance, Dodge, and stat boost.
That's just druid and bard spells, so still plenty more that could be used for inspiration. I also quite like a number of Epic Encounters spells, might ask Sandpie if I can implement them or variations of them.