Paper bomb Jutsu(Ex): With a whisper and swift hand movements, you can infuse a thin, blank, piece of paper with ki to scribe explosive kanji onto its surface as a standard action. The resulting explosion deals 1d6 points of fire damage to all objects within the blast radius of 15 ft per ki point. A ninja can increase the explosive power of the trap by infusing the paper with more ki; the Ninja may infuse an extra point of ki at levels 4, 6, 8, and 10. The devious trap can be placed on any surface that would normally take to an adhesive sticker. The explosive runes detonate when an object moves within 10ft of the trap.
Sure that's fair. Wizards get Fireball at level 5 and it is superior to this version(30ft blast radius with 5d6dmg) so the spellcasters of your group shouldn't feel cheated.
A dagger deals 1d4 dmg, crits on a natural 19-20 deal in x2 DMG. It is arguable that a ninja could throw a knife accurately up to 20-30 ft, because he is throwing the object you would apply your str modifier.
So the run down...
1) Spend ki to make paper bomb.
2)Wrap it around the handle.
3) Roll a hit check to throw dagger. If you hit, roll DMG for the dagger which is 1d4 DMG plus str modifier.
4) Explosion takes place after dagger impact. Roll damage per your level.
5)What if the dagger misses? Everyone or thing takes half damage if they are in the blast zone. Or none if they have "Evasion"
Isn't there a ninja trick where you imbue ki into a thrown weapon and it explodes on impact? You could just reflavor that.