I've got quite a few ideas, and it's something I enjoy fooling with for fun. It's pretty challenging to try making something new that isn't basically a re-hash of an already published class/prestige class.
Here's a sample of things I've come up with in the past:
Burly Brawler: For lack of a better name. Basically a giant hammer of a fighter who does only one thing but does it very well - Strength. Sort of a Monk-type, specializing in unarmed strikes, but without all the fancy Dexterity stuff.
Anatomist: Like a Warlock, but focused on transmutation-like effects and shapechanging. Similar to the Master Transmuter prestige class.
Scientist: I've been having trouble with this one. The idea was sort of like an anti-wizard. Someone who used the powers of reason and logic as mystical effects that would cancel not only magic but supernatural abilities of monsters while providing party members with information based on the powers of observation.
Georus: A semi-spellcaster class similar to the way rangers are spellcasters. Their specialty is using terrain to a tactical and magical advantage.
Umbral Adept: A spellcaster who shapes his spells into semi-permanent ectoplasm creatures that attack for him.
Shield Knight: Martial class with a focus on becoming indestructible.
I also thought of an unnamed prestige class who, when they were dealt damage, would magically deal an equal amount of damage to a chosen foe.
Just recently, I was thinking of interesting, crazy new ways to make a series of arcane school-focused spellcasters. My most recent inspiration was to try translating some common super heroes' abilities into either prestige classes or magic items (like the Green Lanter's ring. Probably a major artifact idea right there).