When building a spellcaster I first think in his role in the party: DEbuffer, Support, blaster, etc. So i found that that thinking always lead me to be pigeonholed to a reduced set of spell which have proved to work great time after time. The thing is that those spells doesn't have much in common (like haste and black tentacles) they just happens to be really good. And it's becoming somewhat boring
So, perhaps I should change the focus, first choose a theme (not a role) and then build around it.
So, what's the best way to build sorcerer/wizards, Magi, oracle/cleric/Warpriest that is a true specialist (in the sense that most of his powers follow the chosen theme) in
- Fire, water (cold), earth or air magic (probably the easier)
- Negative energy
- Holy magic (not just divine spells, but stuffs like holy smite)