The Unforgivables and other evil spells
Avada Kedavra
The killing curse is supposedly painless, unlike many other ways of killing, so what makes it unforgivable?
Does the complete separation of body and soul prevent ghosts from forming?
Is it a necessary component of creating a horcrux?
Armor
There is no known magical block of the AK and as such, physical blocks are the best defense.
So how about actual armour? Would plate armour or modern armour like Kevlar be more effective? Use flexibility and featherlight runes to improve agility, remove weak points/patch over them with magic, use runes to channel magic and/or collect blood/energy/magic in the surrounding area to fuel the wearer
How about basilisk hide/scales or the hide of any other magic resistant species? [How about the hide of animals associated with rebirth, life, death?]
Carrying/conjuring shields (generally, and as a method in impromptu fights)
Is the efficacy influenced by what it is perceived as? Meaning, if you consider it part of you, will it be considered clothing and therefore useless, or still work as defensive armour? Would it work once and then shatter as stone cover has been observed to when hit with the curse? Or would it absorb/deflect the energy and stay?
Obliviation
What are the limits? Could you make someone forget muscle memory? How to speak, walk, take care of themselves?
Could erasing a select number of memories "create" a new person? And if so, what are the ethical implications of such a spell being permissible?
If you erased vast extents of someone's memory and convinced them they were someone else and they don't find out for years, who are they really? The person they originally were or the one you made them? And what effect does that have on their magic and their skills?
If you were to obliviate the witness to a crime, would Veritaserum overpower the obliviation or not?
Can you obliviate a painting? Or a snake (would you need parseltongue for that?)
Love potions
These potions are essentially a targeted version of the Imperius (and apparently bring with them the risk of birthing someone incapable of love so..)
Rituals
Magic theft
Considering the whinging of purebloods about muggleborn, there should be an actual way for wixen to steal the magic of other wixen. Have it come out the wix as a golden light. Like Doctor Who regeneration, but bad