Wrong forum -_-. Move it to the Challenge Section.
Anyway, some Theme suggestions.
Terrain Dependant Spell- Spells that have different effect depending on the terrain.
Astral Spell- A spell related to the Astral theme
Summoning Spell- Invent and alternative way to summon stuff
Action Triggered Spells- Spells that are triggered by certain actions (triggered by spell cast, death etc.)
Aura- Self-explanatory
Remake a Spell- Create an alternative version of a spell that is already existant in
WC3. Example remake storm bolt into a spell that will create a thunder clap when the hammer hits etc.
Something like that :P