I think Door of Destinies is a good choice, but he has better choices. I run a similar deck, except I splash blue for counters and to use blue permanents (Zealous Guardians and Fieldmist Borderposts to be exact) to trigger Mirror-Sigil Sergeant's copy ability. Dropping him on turn 4 with Preeminent Captain works pretty well.
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Sorry dude, but everyone else is right. Mossbridge Troll is a Shadowmoor card and Type 2 legal is the last 2 BLOCKS plus the last core set. The most recent coreset is Magic 2010, and the last 2 blocks are the Alara block (which contains Shards of Alara, Conflux, and Alara Reborn) and the Lorwyn block (which contains Lorwyn, Eventide, Morningtide, and Shadowmoor). Once Zendikar comes out in October, the Lorwyn BLOCK will be cycled out as the Zendikar block is cycled in. But until then, Lorwyn block is Type 2 legal. Also, Type 2 = Standard. In case that confused you too, it did that to me for awhile. And, if you control a creature of power 10 or greater, you can tap it as an instant BEFORE the Aria checks the field for a 20+ creature; whereas you'd have to wait another turn if you just used the Cradle + Aria.
Out of curiosity...Font of Mythos?
I very much like it. Good job.
I always considered a ninja deck to be a deck that burned with creatures instead of red spells. If you don't win by mid-game your in bad shape, but it usually goes off fast like a burn deck. At least that's how I built mine.
I ran a ninja deck too back when it was standard legal. Run a full playset of Ornithopters for sure....they are by far a ninja's best friend since you can redrop them after they've been substituted with ninjitsu in Main Phase 2. Also Higure (the legendary blue ninja) helps you find your other ninjas and adapt to the situation. I also found a playset of Will-o-the-Wisp to be very useful if you don't have an Ornithopter in hand since it costs only 1 black to play, and it can block for you and regen. Lastly, Ronin Warclub is a-freakin-mazing in ninja decks since once a ninja comes in via ninjitsu, the club immediately attaches to them and comes in for more damage. Then redrop the Ornithopter or Will-o-the-Wisp and then your opponent has a 2/3 flying or 2/2 flying regen blocker to deal with. Rinse and repeat :D.